Display device and electronic device including the same

ABSTRACT

It is an object to decrease the number of transistors connected to a capacitor. In a structure, a capacitor and one transistor are included, one electrode of the capacitor is connected to a wiring, and the other electrode of the capacitor is connected to a gate of the transistor. Since a clock signal is input to the wiring, the clock signal is input to the gate of the transistor through the capacitor. Then, on/off of the transistor is controlled by a signal which synchronizes with the clock signal, so that a period when the transistor is on and a period when the transistor is off are repeated. In this manner, deterioration of the transistor can be suppressed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/552,547, filed Nov. 25, 2014, now allowed, which is a continuation ofU.S. application Ser. No. 13/769,999, filed Feb. 19, 2013, now U.S. Pat.No. 8,902,374, which is a continuation of U.S. application Ser. No.12/614,852, filed Nov. 9, 2009, now U.S. Pat. No. 8,902,144, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2008-304124 on Nov. 28, 2008, all of which are incorporatedby reference.

DESCRIPTION

1. Technical Field

The present invention relates to a semiconductor device, a displaydevice, a liquid crystal display device, a driving method thereof, or aproducing method thereof. In specific, the present invention relates toa semiconductor device, a display device, or a liquid crystal displaydevice including a driver circuit formed over the same substrate as apixel portion, or a driving method of the device. Alternatively, thepresent invention relates to an electronic device including the device.

2. Background Art

In recent years, with the increase of large display devices such asliquid crystal televisions, display devices have been activelydeveloped. In specific, a technique of forming a driver circuit such asa gate driver over the same substrate as a pixel portion by using atransistor formed using a non-single-crystal semiconductor has beenactively developed because the technique makes a great contribution forreduction in cost and improvement in reliability.

However, deterioration such as increase in threshold voltage or decreasein mobility is caused in the transistor formed using thenon-single-crystal semiconductor. As the deterioration of the transistoradvances, there is a problem in that the driver circuit becomes hard tooperate and an image cannot be displayed. Accordingly, Patent Document 1discloses a structure of a shift register which can suppress thedeterioration of the transistor. In Patent Document 1, one electrode ofa capacitor is connected to a wiring to which a clock signal is inputand the other electrode of the capacitor is connected to gates of twotransistors, so that the potential of the other electrode of thecapacitor is increased or decreased by making the potential synchronizewith the clock signal. In this manner, by utilizing capacitive couplingof the capacitor, signals that synchronize with the clock signal aregenerated in the gates of the two transistors. Then, by using thesignals that synchronize with the clock signal, on and off of thetransistors is controlled. Accordingly, since a period when thetransistor is on and a period when the transistor is off are repeated,the deterioration of the transistors can be suppressed.

REFERENCE [Patent Document 1] Japanese Published Patent Application No.2006-24350

However, in Patent Document 1, since the other electrode of thecapacitor is connected to the gates of the two transistors, there is aproblem in that the parasitic capacitance of a node connected to thecapacitor is high. Accordingly, there is a problem in that the potentialin an H level of a signal that synchronizes with a clock signal becomeslow. In that case, there is a problem in that a time during which atransistor can be turned is shortened if the threshold voltage of thetransistor increases. That is, there is a problem in that the life of ashift register is shortened. Alternatively, since the parasiticcapacitance of the node connected to the capacitor is high, there is aproblem in that the capacitance value of the capacitor should be large.Accordingly, since an area where the one electrode of the capacitor andthe other electrode of the capacitor overlap with each other needs to belarge, there is a problem in that the layout area of the capacitorbecomes large.

In Patent Document 1, since the area of the capacitor needs to be large,there is a problem in that short circuit between the one electrode andthe other electrode tends to be caused due to dust or the like. As aresult, there is a problem in that yield is decreased and cost isincreased.

In Patent Document 1, since the capacitance value of the capacitor needsto be large, there is a problem in that delay or distortion of a signal(e.g., a clock signal or an inverted clock signal) supplied to thecapacitor becomes obvious. Alternatively, there is a problem in thatpower consumption is increased.

Since a circuit having high current driving capability is used as acircuit for outputting a signal to be supplied to the capacitor, thereis a problem in that an outside circuit (hereinafter also referred to asan external circuit) becomes large. Alternatively, there is a problem inthat a display device becomes large.

In Patent Document 1, a period when a gate of a pull-up transistor Tu isin a floating state exists. Accordingly, noise or the like is causedbecause the potential of the gate of the pull-up transistor Tu is notstable. Therefore, there is a problem in that the shift registermalfunctions.

In view of the foregoing problems, it is an object to decrease thenumber of transistors connected to a capacitor. Alternatively, it is anobject to decrease the parasitic capacitance of a transistor connectedto the capacitor. Alternatively, it is an object to increase thepotential in an H level of a signal which synchronizes with a clocksignal. Alternatively, it is an object to decrease a layout area.Alternatively, it is an object to extend life. Alternatively, it is anobject to decrease delay or distortion of a signal. Alternatively, it isan object to reduce power consumption. Alternatively, it is an object todecrease the adverse effect of noise. Alternatively, it is an object tosuppress or relieve deterioration of a transistor. Alternatively, it isan object to suppress malfunction. Alternatively, it is an object toprevent short circuit between one electrode of a capacitor and the otherelectrode of the capacitor. Alternatively, it is an object to decreasethe current driving capability of an outside circuit. Alternatively, itis an object to reduce the size of an outside circuit. Alternatively, itis an object to reduce the size of a display device. Note that thedescriptions of these problems do not disturb the existence of otherproblems.

DISCLOSURE OF INVENTION

In a structure, a capacitor and one transistor are included, oneelectrode of the capacitor is connected to a wiring, and the otherelectrode of the capacitor is connected to a gate of the transistor.Since a clock signal is input to the wiring, the clock signal is inputto the gate of the transistor through the capacitor. Then, on/off of thetransistor is controlled by a signal which synchronizes with the clocksignal, so that a period when the transistor is on and a period when thetransistor is off are repeated. In this manner, deterioration of thetransistor can be suppressed.

According to one exemplary embodiment of the present invention, a liquidcrystal display device includes a driver circuit and a pixel. The pixelincludes a liquid crystal element. The driver circuit includes a firsttransistor, a second transistor, a third transistor, a fourthtransistor, and a capacitor. A first terminal of the first transistor iselectrically connected to a first wiring. A second terminal of the firsttransistor is electrically connected to a second wiring. A firstterminal of the second transistor is electrically connected to thesecond wiring. A second terminal of the second transistor iselectrically connected to a gate of the first transistor. A gate of thesecond transistor is electrically connected to the first wiring. A firstterminal of the third transistor is electrically connected to a thirdwiring. A second terminal of the third transistor is electricallyconnected to the gate of the first transistor. A first terminal of thefourth transistor is electrically connected to the third wiring. Asecond terminal of the fourth transistor is electrically connected to agate of the third transistor. A gate of the fourth transistor iselectrically connected to the gate of the first transistor. Oneelectrode of the capacitor is electrically connected to the firstwiring. The other electrode of the capacitor is electrically connectedto the gate of the third transistor.

Note that a variety of switches can be used as a switch. For example, anelectrical switch, a mechanical switch, or the like can be used. Thatis, any element can be used as long as it can control a current flow,without limitation to a certain element. For example, a transistor(e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PNdiode, a PIN diode, a Schottky diode, an MIM (metal insulator metal)diode, an MIS (metal insulator semiconductor) diode, or adiode-connected transistor), or the like can be used as a switch.Alternatively, a logic circuit in which such elements are combined canbe used as a switch.

An example of a mechanical switch is a switch formed using a MEMS (microelectro mechanical system) technology, such as a digital micromirrordevice (DMD).

Note that a CMOS switch may be used as a switch by using both ann-channel transistor and a p-channel transistor.

Note that when it is explicitly described that “A and B are connected”,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein. Here, each of A and B is an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer). Accordingly, another elementmay be interposed between elements having a connection relationillustrated in drawings and texts, without limitation to a predeterminedconnection relation, for example, the connection relation illustrated inthe drawings and the texts.

For example, in the case where A and B are electrically connected, oneor more elements which enable electrical connection between A and B(e.g., a switch, a transistor, a capacitor, an inductor, a resistor,and/or a diode) may be connected between A and B. Alternatively, in thecase where A and B are functionally connected, one or more circuitswhich enable functional connection between A and B (e.g., a logiccircuit such as an inverter, a NAND circuit, or a NOR circuit; a signalconverter circuit such as a DA converter circuit, an AD convertercircuit, or a gamma correction circuit; a potential level convertercircuit such as a power supply circuit (e.g., a dc-dc converter, astep-up dc-dc converter, or a step-down dc-dc converter) or a levelshifter circuit for changing a potential level of a signal; a voltagesource; a current source; a switching circuit; an amplifier circuit suchas a circuit which can increase signal amplitude, the amount of current,or the like, an operational amplifier, a differential amplifier circuit,a source follower circuit, or a buffer circuit; a signal generationcircuit; a memory circuit; and/or a control circuit) may be connectedbetween A and B. For example, in the case where a signal output from Ais transmitted to B even when another circuit is interposed between Aand B, A and B are functionally connected.

Note that when it is explicitly described that “A and B are electricallyconnected”, the case where A and B are electrically connected (i.e., thecase where A and B are connected with another element or another circuitinterposed therebetween), the case where A and B are functionallyconnected (i.e., the case where A and B are functionally connected withanother circuit interposed therebetween), and the case where A and B aredirectly connected (i.e., the case where A and B are connected withoutanother element or another circuit interposed therebetween) are includedtherein.

That is, when it is explicitly described that “A and B are electricallyconnected”, the description is the same as the case where it isexplicitly only described that “A and B are connected”.

Note that a display element, a display device which is a deviceincluding a display element, a light-emitting element, and alight-emitting device which is a device including a light-emittingelement can employ various modes and can include various elements. Forexample, a display medium, whose contrast, luminance, reflectivity,transmittance, or the like changes by electromagnetic action, such as anEL (electroluminescence) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor which emits light depending on theamount of current), an electron emitter, a liquid crystal element,electronic ink, an electrophoretic element, a grating light valve (GLV),a plasma display panel (PDP), a digital micromirror device (DMD), apiezoelectric ceramic display, or a carbon nanotube can be used as adisplay element, a display device, a light-emitting element, or alight-emitting device. Note that display devices having EL elementsinclude an EL display; display devices having electron emitters includea field emission display (FED), an SED-type flat panel display (SED:surface-conduction electron-emitter display), and the like; displaydevices having liquid crystal elements include a liquid crystal display(e.g., a transmissive liquid crystal display, a transflective liquidcrystal display, a reflective liquid crystal display, a direct-viewliquid crystal display, or a projection liquid crystal display); displaydevices having electronic ink or electrophoretic elements includeelectronic paper.

Note that a liquid crystal element is an element which controlstransmission or non-transmission of light by optical modulation actionof liquid crystals and includes a pair of electrodes and liquidcrystals. Note that the optical modulation action of liquid crystals iscontrolled by an electric filed applied to the liquid crystals(including a horizontal electric field, a vertical electric field, and adiagonal electric field). Note that the following can be used for aliquid crystal element: a nematic liquid crystal, a cholesteric liquidcrystal, a smectic liquid crystal, a discotic liquid crystal, athermotropic liquid crystal, a lyotropic liquid crystal, a low-molecularliquid crystal, a high-molecular liquid crystal, a polymer dispersedliquid crystal (PDLC), a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a main-chain liquid crystal, aside-chain high-molecular liquid crystal, a plasma addressed liquidcrystal (PALC), a banana-shaped liquid crystal, and the like. Inaddition, the following can be used as a diving method of a liquidcrystal: a TN (twisted nematic) mode, an STN (super twisted nematic)mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching)mode, an MVA (multi-domain vertical alignment) mode, a PVA (patternedvertical alignment) mode, an ASV (advanced super view) mode, an ASM(axially symmetric aligned microcell) mode, an OCB (opticallycompensated birefringence) mode, an ECB (electrically controlledbirefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersedliquid crystal) mode, a guest-host mode, a blue phase mode, and thelike. Note that the present invention is not limited to this, and avariety of liquid crystal elements and driving methods thereof can beused as a liquid crystal element and a driving method thereof.

Note that electroluminescence, a cold cathode fluorescent lamp, a hotcathode fluorescent lamp, an LED, a laser light source, a mercury lamp,or the like can be used as a light source of a display device in which alight source is needed, such as a liquid crystal display (e.g., atransmissive liquid crystal display, a transflective liquid crystaldisplay, a reflective liquid crystal display, a direct-view liquidcrystal display, or a projection liquid crystal display), a displaydevice including a grating light valve (GLV), or a display deviceincluding a digital micromirror device (DMD). Note that the presentinvention is not limited to this, and a variety of light sources can beused as a light source.

Note that a variety of transistors can be used as a transistor, withoutlimitation to a certain type. For example, a thin film transistor (TFT)including a non-single-crystal semiconductor film typified by amorphoussilicon, polycrystalline silicon, microcrystalline (also referred to asmicrocrystal, nanocrystal, or semi-amorphous) silicon, or the like canbe used.

Note that by using a catalyst (e.g., nickel) in the case of formingmicrocrystalline silicon, crystallinity can be further improved and atransistor having excellent electrical characteristics can be formed. Inthis case, crystallinity can be improved by just performing heattreatment without performing laser irradiation. Accordingly, a gatedriver circuit (e.g., a scan line driver circuit) and part of a sourcedriver circuit (e.g., an analog switch) can be formed using the samesubstrate as a pixel portion. In addition, in the case of not performinglaser irradiation for crystallization, unevenness in crystallinity ofsilicon can be suppressed. Therefore, high-quality images can bedisplayed.

Note that polycrystalline silicon and microcrystalline silicon can beformed without using a catalyst (e.g., nickel).

A transistor can be formed using a semiconductor substrate, an SOIsubstrate, or the like. Thus, a transistor with few variations incharacteristics, sizes, shapes, or the like, with high current supplycapability, and with a small size can be formed. By using such atransistor, power consumption of a circuit can be reduced or a circuitcan be highly integrated.

A transistor including a compound semiconductor or an oxidesemiconductor, such as ZnO, a-InGaZnO, SiGe, GaAs, IZO, ITO, or SnO, athin film transistor obtained by thinning such a compound semiconductoror an oxide semiconductor, or the like can be used. Thus, manufacturingtemperature can be lowered and for example, such a transistor can beformed at room temperature. Accordingly, the transistor can be formeddirectly on a substrate having low heat resistance, such as a plasticsubstrate or a film substrate. Note that such a compound semiconductoror an oxide semiconductor can be used not only for a channel portion ofthe transistor but also for other applications. For example, such acompound semiconductor or an oxide semiconductor can be used for aresistor, a pixel electrode, or a light-transmitting electrode. Further,since such an element can be formed at the same time as the transistor,cost can be reduced.

A transistor or the like formed by an inkjet method or a printing methodcan be used. Thus, a transistor can be formed at room temperature, canbe formed at a low vacuum, or can be formed using a large substrate.Since the transistor can be formed without using a mask (reticle), thelayout of the transistor can be easily changed. Further, since it is notnecessary to use a resist, material cost is reduced and the number ofsteps can be reduced. Furthermore, since a film is formed only in anecessary portion, a material is not wasted as compared to amanufacturing method by which etching is performed after the film isformed over the entire surface, so that cost can be reduced.

A transistor or the like including an organic semiconductor or a carbonnanotube can be used. Thus, such a transistor can be formed over aflexible substrate. A semiconductor device formed using such a substratecan resist shocks.

Further, transistors with a variety of structures can be used. Forexample, a MOS transistor, a junction transistor, a bipolar transistor,or the like can be used as a transistor. By using a MOS transistor, thesize of the transistor can be reduced. Thus, a large number oftransistors can be mounted. By using a bipolar transistor, large currentcan flow. Thus, a circuit can be operated at high speed.

Note that a MOS transistor, a bipolar transistor, and the like may beformed over one substrate. Thus, reduction in power consumption,reduction in size, high-speed operation, and the like can be achieved.

Furthermore, a variety of transistors can be used.

Note that a transistor can be formed using a variety of substrates,without limitation to a certain type. For example, a single crystalsubstrate, an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a stainless steel substrate, a substrate including astainless steel foil, or the like can be used as a substrate.

Note that the structure of a transistor can be a variety of structures,without limitation to a certain structure. For example, a multi-gatestructure having two or more gate electrodes can be used. By using themulti-gate structure, a structure where a plurality of transistors areconnected in series is provided because channel regions are connected inseries.

As another example, a structure where gate electrodes are formed aboveand below a channel can be used. Note that when the gate electrodes areformed above and below the channel, a structure where a plurality oftransistors are connected in parallel is provided.

A structure where a gate electrode is formed above a channel region, astructure where a gate electrode is formed below a channel region, astaggered structure, an inverted staggered structure, a structure wherea channel region is divided into a plurality of regions, or a structurewhere channel regions are connected in parallel or in series can beused. Alternatively, a structure where a source electrode or a drainelectrode overlaps with a channel region (or part of it) can be used.Further, an LDD region may be provided.

Note that a variety of transistors can be used as a transistor, and thetransistor can be formed using a variety of substrates. Accordingly, allthe circuits that are necessary to realize a predetermined function canbe formed using the same substrate. For example, all the circuits thatare necessary to realize the predetermined function can be formed usinga glass substrate, a plastic substrate, a single crystal substrate, anSOT substrate, or any other substrate. Alternatively, some of thecircuits which are necessary to realize the predetermined function canbe formed using one substrate and some of the circuits which arenecessary to realize the predetermined function can be formed usinganother substrate. That is, not all the circuits that are necessary torealize the predetermined function are required to be formed using thesame substrate. For example, some of the circuits which are necessary torealize the predetermined function can be formed by transistors using aglass substrate and some of the circuits which are necessary to realizethe predetermined function can be formed using a single crystalsubstrate, so that an IC chip formed by a transistor using the singlecrystal substrate can be connected to the glass substrate by COG (chipon glass) and the IC chip may be provided over the glass substrate.Alternatively, the IC chip can be connected to the glass substrate byTAB (tape automated bonding) or a printed wiring board. Alternatively,when circuits with high driving voltage and high driving frequency,which consume large power, are formed using a single crystal substrateinstead of forming such circuits using the same substrate, and an ICchip formed by the circuits is used, for example, increase in powerconsumption can be prevented.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor has a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor change depending on thestructure, the operating condition, and the like of the transistor, itis difficult to define which is a source or a drain. Thus, a regionwhich functions as a source and a drain is not referred to as a sourceor a drain in some cases. In such a case, one of the source and thedrain might be referred to as a first terminal and the other of thesource and the drain might be referred to as a second terminal, forexample. Alternatively, one of the source and the drain might bereferred to as a first electrode and the other of the source and thedrain might be referred to as a second electrode. Alternatively, one ofthe source and the drain might be referred to as a first region and theother of the source and the drain might be referred to as a secondregion.

Note that a transistor may be an element having at least three terminalsof a base, an emitter, and a collector. In this case, in a similarmanner, one of the emitter and the collector might be referred to as afirst terminal and the other of the emitter and the collector might bereferred to as a second terminal.

Note that a semiconductor device corresponds to a device having acircuit including a semiconductor element (e.g., a transistor, a diode,or a thyristor). The semiconductor device may also correspond to alldevices that can function by utilizing semiconductor characteristics. Inaddition, the semiconductor device corresponds to a device having asemiconductor material.

Note that a display device corresponds to a device having a displayelement. The display device may include a plurality of pixels eachhaving a display element. Note that the display device may include aperipheral driver circuit for driving the plurality of pixels. Note thatthe peripheral driver circuit for driving the plurality of pixels may beformed using the same substrate as the plurality of pixels. The displaydevice may include a peripheral driver circuit provided over a substrateby wire bonding or bump bonding, namely, an IC chip connected by chip onglass (COG) or an IC chip connected by TAB or the like. The displaydevice may include a flexible printed circuit (FPC) to which an IC chip,a resistor, a capacitor, an inductor, a transistor, or the like isattached. Note that the display device may include a printed wiringboard (PWB) which is connected through a flexible printed circuit (FPC)and to which an IC chip, a resistor, a capacitor, an inductor, atransistor, or the like is attached. The display device may include anoptical sheet such as a polarizing plate or a retardation plate. Thedisplay device may include a lighting device, a housing, an audio inputand output device, an optical sensor, or the like.

Note that a lighting device may include a backlight unit, a light guideplate, a prism sheet, a diffusion sheet, a reflective sheet, a lightsource (e.g., an LED or a cold cathode fluorescent lamp), a coolingdevice (e.g., a water cooling device or an air cooling device), or thelike.

Note that a light-emitting device corresponds to a device having alight-emitting element or the like. In the case where a light-emittingdevice includes a light-emitting element as a display element, thelight-emitting device is one of specific examples of a display device.

Note that a reflective device corresponds to a device having alight-reflective element, a light diffraction element, light-reflectiveelectrode, or the like.

Note that a liquid crystal display device corresponds to a displaydevice including a liquid crystal element. Liquid crystal displaydevices include a direct-view liquid crystal display, a projectionliquid crystal display, a transmissive liquid crystal display, areflective liquid crystal display, a transflective liquid crystaldisplay, and the like.

Note that a driving device corresponds to a device having asemiconductor element, an electric circuit, or an electronic circuit.For example, a transistor which controls input of signals from a sourcesignal line to pixels (also referred to as a selection transistor, aswitching transistor, or the like), a transistor which supplies voltageor current to a pixel electrode, a transistor which supplies voltage orcurrent to a light-emitting element, and the like are examples of thedriving device. A circuit which supplies signals to a gate signal line(also referred to as a gate driver, a gate line driver circuit, or thelike), a circuit which supplies signals to a source signal line (alsoreferred to as a source driver, a source line driver circuit, or thelike), and the like are also examples of the driving device.

Note that a display device, a semiconductor device, a lighting device, acooling device, a light-emitting device, a reflective device, a drivingdevice, and the like overlap with each other in some cases. For example,a display device includes a semiconductor device and a light-emittingdevice in some cases. Alternatively, a semiconductor device includes adisplay device and a driving device in some cases.

Note that when it is explicitly described that “B is formed on A” or “Bis formed over A”, it does not necessarily mean that B is formed indirect contact with A. The description includes the case where A and Bare not in direct contact with each other, i.e., the case where anotherobject is interposed between A and B. Here, each of A and B is an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer).

Accordingly, for example, when it is explicitly described that “a layerB is formed on (or over) a layer A”, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or the layer D. Note that another layer (e.g., a layer C or alayer D) may be a single layer or a plurality of layers.

In a similar manner, when it is explicitly described that “B is formedabove A”, it does not necessarily mean that B is formed in directcontact with A, and another object may be interposed therebetween. Thus,for example, when it is described that “a layer B is formed above alayer A”, it includes both the case where the layer B is formed indirect contact with the layer A, and the case where another layer (e.g.,a layer C or a layer D) is formed in direct contact with the layer A andthe layer B is formed in direct contact with the layer C or the layer D.Note that another layer (e.g., a layer C or a layer D) may be a singlelayer or a plurality of layers.

Note that when it is explicitly described that “B is formed on A”, “B isformed over A”, or “B is formed above A”, it includes the case where Bis formed obliquely over/above A.

Note that the same can be said when it is described that “B is formedbelow A” or “B is formed under A”.

Note that when an object is explicitly described in a singular form, theobject is preferably singular. Note that the present invention is notlimited to this, and the object can be plural. In a similar manner, whenan object is explicitly described in a plural form, the object ispreferably plural. Note that the present invention is not limited tothis, and the object can be singular.

Note that size, the thickness of layers, or regions in diagrams areexaggerated for simplicity in some cases. Therefore, the presentinvention is not necessarily limited to the scale.

Note that diagrams are schematic views of ideal examples, and shapes orvalues are not limited to those illustrated in the diagrams. Forexample, it is possible to include variations in shape due to amanufacturing technique or an error, variations in signals, voltagevalues, or current values due to noise or a difference in a timing.

Note that a technical term is used in order to describe a particularembodiment or example or the like in many cases, and is not limited tothis.

Note that terms which are not defined (including terms used for scienceand technology, such as technical terms or academic parlance) can beused as terms which have meaning equal to general meaning that anordinary person skilled in the art understands. It is preferable thatterms defined by dictionaries or the like be construed as consistentmeaning with the background of related art.

Note that terms such as “first”, “second”, “third”, and the like areused for distinguishing various elements, members, regions, layers, andareas from others. Therefore, the terms such as “first”, “second”,“third”, and the like do not limit the number of the elements, members,regions, layers, areas, or the like. Further, for example, “first” canbe replaced with “second”, “third”, or the like.

The number of transistors connected to a capacitor can be decreased.Alternatively, the parasitic capacitance of a transistor connected tothe capacitor can be decreased. Alternatively, the potential in an Hlevel of a signal which synchronizes with a clock signal can beincreased. Alternatively, a layout area can be reduced. Alternatively,life can be extended. Alternatively, delay or distortion of a signal canbe decreased. Alternatively, power consumption can be decreased.Alternatively, the adverse effect of noise can be decreased.Alternatively, deterioration of a transistor can be suppressed orrelieved. Alternatively, malfunction can be suppressed. Alternatively,short circuit between one electrode of a capacitor and the otherelectrode of the capacitor can be prevented. Alternatively, the currentdriving capability of an outside circuit can be decreased.Alternatively, the size of an outside circuit can be reduced.Alternatively, the size of a display device can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a circuit diagram of a semiconductor device and FIG. 1B is atiming chart for illustrating a driving method of the semiconductordevice.

FIGS. 2A to 2E are schematic views illustrating a driving method of asemiconductor device.

FIGS. 3A to 3E are circuit diagrams of a semiconductor device.

FIGS. 4A to 4F are circuit diagrams of a semiconductor device.

FIGS. 5A to 5E are circuit diagrams of a semiconductor device.

FIG. 6A is a circuit diagram of a semiconductor device and FIGS. 6B and6C are timing charts each illustrating a driving method of thesemiconductor device.

FIGS. 7A to 7C are schematic views illustrating a driving method of asemiconductor device.

FIGS. 8A and 8B are schematic views illustrating a driving method of asemiconductor device.

FIGS. 9A to 9C are circuit diagrams of a semiconductor device.

FIGS. 10A to 10C are circuit diagrams of a semiconductor device.

FIGS. 11A to 11C are circuit diagrams of a semiconductor device.

FIGS. 12A to 12C are circuit diagrams of a semiconductor device.

FIGS. 13A to 13C are circuit diagrams of a semiconductor device.

FIG. 14A is a circuit diagram of a shift register and FIG. 14B is atiming chart illustrating a driving method of the shift register.

FIG. 15 is a circuit diagram of a shift register.

FIG. 16 is a circuit diagram of a shift register.

FIGS. 17A and 17B are circuit diagrams of a shift register.

FIG. 18 is a layout view of a shift register.

FIG. 19A is a circuit diagram of a semiconductor device and FIG. 19B isa timing chart illustrating a driving method of the semiconductordevice.

FIGS. 20A and 20B are circuit diagrams of a semiconductor device.

FIG. 21 is a circuit diagram of a shift register.

FIGS. 22A and 22B are system block diagrams of a display device.

FIGS. 23A to 23E are diagrams each illustrating a structure of a displaydevice.

FIG. 24 is a circuit diagram of a shift register.

FIGS. 25A and 25B are timing charts each illustrating a driving methodof a shift register.

FIG. 26A is a circuit diagram of a signal line driver circuit and FIG.26B is a timing chart illustrating a driving method of the signal linedriver circuit.

FIGS. 27A to 27C, 27E, and 27F are circuit diagrams of a pixel and FIGS.27D and 27G are timing charts each illustrating a driving method of thepixel.

FIGS. 28A and 28B are circuit diagrams of a pixel, FIGS. 28C to 28E and28G are layout diagrams of the pixel, and FIGS. 28F and 28H are timingcharts each illustrating a driving method of the pixel.

FIG. 29A is a timing chart illustrating a driving method of a pixel andFIG. 29B is a circuit diagram of the pixel.

FIG. 30 is a layout view of a shift register.

FIG. 31 is a layout view of a shift register.

FIGS. 32A to 32C are cross-sectional views of a transistor.

FIGS. 33A to 33H are diagrams illustrating electronic devices.

FIGS. 34A to 34H are diagrams illustrating electronic devices.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to thedrawings. However, the embodiments can be implemented with variousmodes. It will be readily appreciated by those skilled in the art thatmodes and details can be changed in various ways without departing fromthe spirit and scope of the present invention. Therefore, the inventionshould not be construed as being limited to the description ofembodiment. Note that in structures of the invention described below,the same portions or portions having similar functions are denoted bythe same reference numerals, and description thereof is not repeated.

Note that a content (or may be part of the content) described in oneembodiment may be applied to, combined with, or replaced by a differentcontent (or may be part of the different content) described in the sameembodiment and/or a content (or may be part of the content) described inone or a plurality of different embodiments.

Note that in each embodiment, a content described in the embodiment is acontent described with reference to a variety of diagrams or a contentdescribed with a paragraph disclosed in this specification.

Note that, by combining a diagram (or may be part of the diagram)described one embodiment with another part of the diagram, a differentdiagram (or may be part of the different diagram) described in the sameembodiment, and/or a diagram (or may be part of the diagram) describedin one or a plurality of different embodiments, much more diagrams canbe formed.

Embodiment 1

In this embodiment, one example of a semiconductor device will bedescribed. Note that the semiconductor device can be denoted as a drivercircuit or a flip-flop.

First, one example of the semiconductor device of this embodiment willbe described with reference to FIG. 1A. The semiconductor device in FIG.1A includes a circuit 100, a transistor 101, a transistor 102, atransistor 103, a transistor 104, a capacitor 105, and a capacitor 106.Each of the transistors 101 to 104 is an n-channel transistor which isturned on when potential difference (Vgs) between a gate and a sourcegets higher than threshold voltage (Vth). However, this embodiment isnot limited to this. Each of the transistors 101 to 104 can be ap-channel transistor. The p-channel transistor is turned on whenpotential difference (Vgs) between a gate and a source gets lower thanthreshold voltage (Vth).

A connection relation of the semiconductor device in FIG. 1A will bedescribed. A first terminal of the transistor 101 is connected to awiring 123B. A second terminal of the transistor 101 is connected to awiring 121. A first terminal of the transistor 102 is connected to agate of the transistor 101. A second terminal of the transistor 102 isconnected to the wiring 121. A gate of the transistor 102 is connectedto a wiring 123C. A first terminal of the transistor 103 is connected toa wiring 122A. A second terminal of the transistor 103 is connected tothe gate of the transistor 101. A first terminal of the transistor 104is connected to a wiring 122B. A second terminal of the transistor 104is connected to the gate of the transistor 103. One electrode of thecapacitor 105 is connected to the gate of the transistor 101. The otherelectrode of the capacitor 105 is connected to the wiring 121. Oneelectrode of the capacitor 106 is connected to the wiring 123A. Theother electrode of the capacitor 106 is connected to the gate of thetransistor 103.

Note that a connection potion of the gate of the transistor 101, thefirst terminal of the transistor 102, the second terminal of thetransistor 103, or the gate of the transistor 104 is denoted as a nodeA. Then, a connection potion of the gate of the transistor 103, thesecond terminal of the transistor 104, or the other electrode of thecapacitor 106 is denoted as a node B. Note that the node A and the nodeB can be denoted as wirings.

Note that the wiring 121, the wiring 123A, the wiring 123B, the wiring123C, the wiring 122A, and the wiring 122B can be denoted as terminals.

One example of a thing (e.g., a signal, voltage, or current) which canbe input to each of the wirings (the wiring 121, the wirings 122A and122B, and the wirings 123A to 123C) is described. However, a contentdescribed below is an example and this embodiment is not limited tothis. A variety of things besides the thing described below can be inputto each of the wirings. Each of the wirings can be in a floating state.

For example, a signal S1 is output from the wiring 121. Accordingly, thewiring 121 can function as a signal line. In specific, in the case wherethe wiring 121 is connected to a pixel, or in the case where the wiring121 is provided so as to extend to a pixel portion, the wiring 121 canfunction as a gate line, a scan line, or a capacitor line. The signal S1is an output signal of the semiconductor device and is a digital signalhaving an H level and an L level in many cases. The signal S1 canfunction as an output signal, a selection signal, a gate signal, or ascan signal.

For example, voltage V1 is supplied to the wirings 122A and 122B.Accordingly, the wirings 122A and 122B can function as power supplylines. The voltage V1 has approximately the same value as the signal S1in an L level in many cases and can function as ground voltage, powersupply voltage, or negative power supply voltage. However, thisembodiment is not limited to this. A signal such as a clock signal canbe input to the wirings 122A and 122B. In that case, the wirings 122Aand 122B can function as signal lines or clock signal lines.Alternatively, different voltages or different signals can be input tothe wirings 122A and 122B.

Note that the term “approximately” means that a value includes a varietyof errors such as an error due to noise, an error due to variations in aprocess, an error due to variations in steps of manufacturing anelement, and/or a measurement error.

For example, a signal S2 is input to the wirings 123A to 123C.Accordingly, the wirings 123A to 123C can function as signal lines. Thesignal S2 is a digital signal which repeatedly switch between an H leveland an L level in a certain cycle in many cases and can function as aclock signal (CK). However, this embodiment is not limited to this.Power supply voltage can be supplied to the wirings 123A to 123C. Inthat case, the wirings 123A to 123C can function as power supply lines.Alternatively, different voltages or different signals can be input tothe wirings 123A to 123C.

Note that in this embodiment, when the potential in an L level of asignal is V1 and the potential in an H level of a signal is V2, forexample, V2 is higher than V1. However, this embodiment is not limitedto this.

Note that voltage means a potential difference between one potential andreference potential (e.g., ground potential) in many cases. Accordingly,voltage, potential and a potential difference can be referred to aspotential, voltage, and a voltage difference, respectively.

Examples of functions which the circuit 100, the transistors 101 to 104,the capacitor 105, and the capacitor 106 have will be described.However, a content described below is one example and this embodiment isnot limited to the content below. The circuit 100 and each element canhave a variety of functions in addition to the functions describedbelow. Alternatively, it is possible that the circuit 100 and eachelement do not have the functions described below.

The circuit 100 has a function of controlling the potential or state ofthe node A and a function of controlling the potential or state of thewiring 121. For example, the circuit 100 has a function of raising thepotential of the node A or the wiring 121, a function of decreasing thepotential of the node A or the wiring 121, and/or a function of makingthe node A or the wiring 121 go into a floating state, or the like. Thetransistor 101 has a function of raising the potential of the wiring 121in accordance with a signal (e.g., the signal S2) which is input to thewiring 123B. The transistor 102 has a function of controlling a timingwhen the wiring 121 and the node A are brought into electricalconduction in accordance with a signal (e.g., the signal S2) which isinput to the wiring 123C and functions as a switch. The transistor 103has a function of controlling a timing when the wiring 122A and the nodeA are brought into electrical conduction in accordance with thepotential of the node B and functions as a switch. The transistor 104has a function of controlling a timing when the wiring 122B and the nodeB are brought into electrical conduction in accordance with thepotential of the node A and functions as a switch. The capacitor 105 hasa function of raising the potential of the node A in accordance with thepotential of the wiring 126 and/or a function of holding a potentialdifference between the gate and the second terminal of the transistor101. The capacitor 106 has a function of controlling the potential ofthe node B in accordance with a signal (e.g., the signal S2) which isinput to the wiring 123A.

Next, operation of the semiconductor device in FIG. 1A will be describedwith reference to FIG. 1B and FIGS. 2A to 2E. FIG. 1B is one example ofa timing chart for illustrating the operation of the semiconductordevice and there are a period T1, a period T2, a period T3, a period T4,and a period T5. In addition, FIG. 1B shows the signal S1, the signalS2, the potential Va of the node A, and the potential Vb of the node B.FIG. 2A is a schematic view of operation of the semiconductor device inFIG. 1A during the period T1. FIG. 2B is a schematic view of theoperation of the semiconductor device in FIG. 1A during the period T2.FIG. 2C is a schematic view of the operation of the semiconductor devicein FIG. 1A during the period T3. FIG. 2D is a schematic view of theoperation of the semiconductor device in FIG. 1A during the period T4.FIG. 2E is a schematic view of the operation of the semiconductor devicein FIG. 1A during the period T5.

Note that when the potential of the node A is raised, the semiconductordevice sequentially performs operation during the period T1, operationduring the period T2, and operation during the period T3. After that,the semiconductor device sequentially repeats operation during theperiod T4 and operation during the period T5 until the potential of thenode A is raised again.

First, the signal S2 goes into an L level in the period T1. Then, thetransistor 102 is turned off, so that the node A and the wiring 121 arebrought out of electrical conduction. At the same time, the potential ofthe node B decreases due to capacitive coupling of the capacitor 106.When the potential of the node B at that time gets lower than the sum ofthe potential (V1) of the wiring 122A and the threshold voltage (Vth106)of the transistor 103, (V1+Vth106), the transistor 103 is turned off.

Accordingly, the wiring 122A and the node A are brought out ofelectrical conduction. On the other hand, the circuit 100 starts toraise the potential of the node A. Then, when the potential of the nodeA becomes equal to the sum of the potential (V1) of the wiring 122B andthe threshold voltage (Vth104) of the transistor 104, (V1+Vth104), thetransistor 104 is turned on. Accordingly, the wiring 122B and the node Bare brought into electrical conduction. Therefore, since the voltage V1is supplied from the wiring 122B to the node B, the potential of thenode B is V1. As a result, since the transistor 103 is kept off, thewiring 122A and the node A are kept out of electrical conduction.Similarly, when the potential of the node A becomes equal to the sum ofthe potential (V1) of the wiring 123B and the threshold voltage (Vth101)of the transistor 101, (V1+Vth101), the transistor 101 is turned on.Accordingly, the wiring 123B and the wiring 121 are brought intoelectrical conduction. Therefore, since the signal S2 in the L level issupplied from the wiring 123B to the wiring 121, the potential of thewiring 121 is approximately equal to the potential of the wiring 123B(the L level of the signal S2 or V1). After that, since the circuit 100stops supplying a signal to the node A when the potential of the node Ais raised to a certain value (e.g., more than or equal to V1+Vth101 andless than or equal to V2), the circuit 100 and the node A are broughtout of electrical conduction. Accordingly, the node A goes into afloating state, so that the potential of the node A is maintained as alarge value. A potential difference between the node A and the wiring121 at that time is held in the capacitor 105.

Note that the circuit 100 can supply the voltage V1, a signal in an Llevel, or the like to the wiring 121 during the period T1.Alternatively, the circuit 100 and the wiring 121 can be brought out ofelectrical conduction if the circuit 100 does not supply the signal orthe like to the wiring 121. In addition, the circuit 100 can make thewiring 121 go into a floating state.

Next, since the potential of the node A is maintained as a large valueduring the period T2, the transistor 104 is kept on. Accordingly, sincethe wiring 122B and the node B are kept in electrical conduction, thepotential of the node B is kept as V1. As a result, the transistor 103is kept off, so that the wiring 122A and the node A are kept out ofelectrical conduction. Similarly, since the potential of the node A ismaintained as a large value, the transistor 101 is kept on. Therefore,the wiring 123B and the wiring 121 are kept in electrical conduction. Atthat time, the level of the signal S2 raises from the L level to an Hlevel. Accordingly, since the wiring 123B and the wiring 121 are kept inelectrical conduction, the potential of the wiring 121 starts to beraised. Since the transistor 102 is turned on at the same time, the nodeA and the wiring 121 are brought into electrical conduction. However,the transistor 102 is turned off when the potential of the wiring 121 israised to a value obtained by subtracting the threshold voltage (Vth102)of the transistor 102 from the potential (V2) of the wiring 123C,(V2−Vth102). Accordingly, the wiring 121 and the node A are brought outof electrical conduction. Here, the capacitor 105 keeps holding thepotential difference between the wiring 121 and the node A in the periodT1. Accordingly, when the potential of the wiring 121 is raised, thepotential of the node A is raised to (V2+Vth101+α) (α is a positivenumber) by capacitive coupling of the capacitor 105. So-called bootstrapoperation is performed. Accordingly, the potential of the wiring 121 israised until it becomes equal to the potential (the H level of thesignal S2 or V2) of the wiring 123B.

Note that since the circuit 100 does not supply a signal or the like tothe node A during the period T2 in many cases, the circuit 100 and thenode A are out of electrical conduction in many cases. In this manner,the circuit 100 makes the node A go into a floating state in many cases.

Note that since the circuit 100 does not supply a signal or the like tothe wiring 121 during the period T2 in many cases, the circuit 100 andthe wiring 121 are out of electrical conduction in many cases.

Next, the circuit 100 decreases the potential of the node A to V1 afterthe level of the signal S2 is dropped from the H level to the L levelduring the period T3. Accordingly, the transistor 101 is on until thepotential of the node A becomes equal to the sum of the potential (V1)of the wiring 123B and the threshold voltage (Vth101) of the transistor101, (V1+Vth101). Accordingly, since the signal S2 in the L level issupplied from the wiring 123B to the wiring 121, the potential of thewiring 121 decreases to the potential (V1) of the wiring 123B.Similarly, the transistor 104 is on until the potential of the node Abecomes equal to the sum of the potential (V1) of the wiring 122B andthe threshold voltage (Vth104) of the transistor 104, (V1+Vth104).

Accordingly, since the voltage V1 is supplied from the wiring 122B tothe node B, the potential of the node B is kept as V1. As a result, thetransistor 103 is kept off, so that the wiring 122A and the node A arekept out of electrical conduction. At that time, the capacitor 106 holdsthe potential difference between the potential of the wiring 123A (the Llevel of the signal S2 or V1) and the potential (V1) of the wiring 122B.

Note that the circuit 100 can supply the voltage V1, a signal in an Llevel, or the like to the wiring 121 during the period T3.Alternatively, the circuit 100 and the wiring 121 can be brought out ofelectrical conduction if the circuit 100 does not supply the signal orthe like to the wiring 121. In addition, the circuit 100 can make thewiring 121 go into a floating state.

Next, the level of the signal S2 is raised from the L level to the Hlevel during the period T4. At that time, since the potential of thenode A is kept as V1, the transistor 101 and the transistor 104 are keptoff. Accordingly, since the node B is kept in a floating state, thepotential of the node B is raised by capacitive coupling of thecapacitor 106. When the potential of the node B gets higher than the sumof the potential (V1) of the wiring 122A and the threshold voltage(Vth103) of the transistor 103, (V1+Vth103), the transistor 103 isturned on. Then, the wiring 122A and the node A are brought intoelectrical conduction. Accordingly, since the voltage V1 is suppliedfrom the wiring 122A to the node A, the potential of the node A ismaintained as V1. At the same time, since the transistor 102 is turnedon, the wiring 121 and the node A are brought into electricalconduction. At that time, the voltage V1 is supplied from the wiring122A to the node A. Accordingly, since the voltage V1 is supplied fromthe wiring 122A to the wiring 121, the potential of the wiring 121 ismaintained as V1.

Note that the circuit 100 can supply the voltage V1, a signal in an Llevel, or the like to the node A during the period T4. Alternatively,the circuit 100 and the wiring 121 can be brought out of electricalconduction if the circuit 100 does not supply the signal or the like tothe node A. In addition, the circuit 100 can make the node A go into afloating state.

Note that the circuit 100 can supply the voltage V1, a signal in an Llevel, or the like to the wiring 121 during the period T5.Alternatively, the circuit 100 and the wiring 121 can be brought out ofelectrical conduction if the circuit 100 does not supply the signal orthe like to the wiring 121. In addition, the circuit 100 can make thewiring 121 go into a floating state.

Next, the level of the signal S2 is dropped from the H level to the Llevel during the period T5. At that time, since the potential of thenode A is kept as V1, the transistor 101 and the transistor 104 are keptoff. Accordingly, the potential of the node B is decreased by capacitivecoupling of the capacitor 106. When the potential of the node B getslower than the sum of the potential (V1) of the wiring 122A and thethreshold voltage (Vth103) of the transistor 103, (V1+Vth103), thetransistor 103 is turned off. Then, the wiring 122A and the node A arebrought out of electrical conduction. Similarly, since the transistor102 is turned off, the wiring 121 and the node A are brought out ofelectrical conduction. At that time, if the circuit 100 supplies thesignal in the L level or the voltage V1 to the node A and the wiring121, the potential of the node A and the potential of the wiring 121 aremaintained as V1. However, even if the circuit 100 does not supply thesignal in the L level or the voltage V1 to the node A and the wiring121, the node A and the wiring 121 go into a floating state, whereby thepotential of the node A and the potential of the wiring 121 aremaintained as V1.

In the semiconductor device in FIG. 1A, the number of transistors thatare connected to the other electrode of the capacitor 106 can be smallerthan that of a conventional technique. Accordingly, the parasiticcapacitance of a node connected to the other of the capacitor 106, thatis, the parasitic capacitance of the node B can be made low. Note thatthe parasitic capacitance means total capacitance such as the gatecapacitance of a transistor, the parasitic capacitance between a gateand source of the transistor, the parasitic capacitance between the gateand drain of the transistor, and/or wiring capacitance. However, thisembodiment is not limited to this. A plurality of transistors can beconnected to the other electrode of the capacitor 106.

Alternatively, since the parasitic capacitance of the node B can bereduced in the semiconductor device shown in FIG. 1A, the capacitancevalue of the capacitor 106 can be made smaller than that of theconventional technique. Accordingly, since an area where one electrodeof the capacitor 106 and the other electrode of the capacitor 106overlap with each other can be reduced, the layout area of the capacitor106 can be reduced. As a result, short circuit between one electrode ofthe capacitor 106 and the other electrode of the capacitor 106 due todust or the like can be prevented. Accordingly, improvement in yield orreduction in cost can be achieved. Alternatively, since the load of thewiring 123A can be reduced, distortion, delay, or the like of a signal(e.g., a signal S2) which is input to the wiring 123A can be suppressed.Alternatively, since the current driving capability of an outsidecircuit for supplying a signal to the wiring 123A can be made low, thesize of the outside circuit can be reduced.

Alternatively, since the parasitic capacitance of the node B can bereduced in the semiconductor device in FIG. 1A, the amplitude voltage ofthe node B in the case where the potential of the wiring 123A is changedcan be made high. Accordingly, during the period T4, since the potentialof the node B can be made higher than that of the conventionaltechnique, the Vgs of the transistor 103 can be made big. That is, sincethe on resistance of the transistor 103 can be made low, the potentialof the node B during the period T4 can be easily maintained as V1.Alternatively, since the channel width (W) of the transistor 103 can bemade small, reduction in a layout area can be achieved.

Alternatively, in the semiconductor device shown in FIG. 1A, during theperiod T2, the node A and the wiring 121 are in electrical conductionuntil the transistor 102 is turned off in many cases. Accordingly, sincethe potential of the node A is decreased, the gate voltage of thetransistor 101 and the transistor 104 can be reduced. As a result,deterioration of characteristics of the transistor 101 and thetransistor 104 can be suppressed. Alternatively, a breakdown of thetransistor 101 and the transistor 104 can be suppressed. Alternatively,a transistor whose mobility is improved by thinning a gate insulatingfilm can be used as the transistor. In the case where such a transistoris used, the channel width (W) of the transistor can be reduced.Accordingly, reduction in a layout area can be achieved.

Alternatively, all transistors in the semiconductor device in FIG. 1Acan be n-channel transistors or p-channel transistors. Accordingly, ascompared to a CMOS circuit, reduction in the number of steps,improvement in yield, or reduction in cost can be achieved. In specific,if all the transistors are n-channel transistors, a non-single-crystalsemiconductor, a microcrystalline semiconductor, an organicsemiconductor, or an oxide semiconductor can be used for a semiconductorlayer of the transistor. Accordingly, reduction in the number of steps,improvement in yield, or reduction in cost can be achieved. However,this embodiment is not limited to this. The semiconductor device shownin FIG. 1A can be formed using a CMOS circuit in which a p-channeltransistor and an n-channel transistor are combined.

Alternatively, in the semiconductor device shown in FIG. 1A, thetransistors 101 to 104 are turned off during at least one of the periodT4 and the period 15. Accordingly, since the transistor is not turned onduring one operation period, deterioration of characteristics of thetransistor, such as increase in threshold voltage or decrease inmobility can be suppressed.

In specific, in the case where the non-single-crystal semiconductor, themicrocrystalline semiconductor, the organic semiconductor, or the oxidesemiconductor is used for the semiconductor layer of the transistor,deterioration of the characteristics of the transistor becomes obvious.However, in the semiconductor device shown in FIG. 1A, deterioration ofthe characteristics of the transistor can be suppressed; therefore, thenon-single-crystal semiconductor, the microcrystalline semiconductor,the organic semiconductor, or the oxide semiconductor can be used forthe semiconductor layer of the transistor. However, this embodiment isnot limited to this. A polycrystalline semiconductor or a single crystalsemiconductor can be used for the semiconductor layer.

Note that the period T2 can be referred to as a selection period andperiods other than the period T2 (the period T1, the period T3, theperiod T4, and the period T5) can be referred to as non-selectionperiods. Alternatively, the period T1, the period T2, the period T3, theperiod T4, and the period T5 can be referred to as a set period, anoutput period, a reset period, a first non-selection period, and asecond non-selection period, respectively.

Note that the channel width (W) of the transistor 101 can be larger thanthat of the transistor 102, the transistor 103, and/or the transistor104. Alternatively, the channel width of the transistor 101 can be thelargest among those of the transistors included in the semiconductordevice. In that case, since the on resistance of the transistor 101 islow, rising time and falling time of a signal (e.g., the signal S1)which is output from the wiring 121 can be shortened. Accordingly,during the period T2, a timing when the transistor 102 is turned offcomes earlier. Therefore, malfunction of the semiconductor device due totoo much reduction in the potential of the node A can be suppressed.However, this embodiment is not limited to this. The channel width ofthe transistor 101 can be smaller than that of any one of thetransistors 102 to 104 or that of any one of the transistors included inthe semiconductor device.

Note that the channel width of the transistor can also be referred to asa W/L ratio of the transistor (L: channel length).

Note that the potential of the signal in an L level which is input tothe wiring 123A, the wiring 123B, and/or the wiring 123C can be lowerthan V1. In that case, backward bias can be applied to the transistor,so that deterioration of the characteristics of the transistor can besuppressed. In specific, since a period of time when the transistor 102is on is long, the potential of the signal in the L level that is inputto the wiring 123C is preferably lower than V1. However, this embodimentis not limited to this. The potential of the signal in the L level thatis input to the wiring 123A, the wiring 123B, and/or the wiring 123C canbe higher than V1.

Note that the potential of the signal in an H level which is input tothe wiring 123A, the wiring 123B, and/or the wiring 123C can be lowerthan V2. In that case, the Vgs of the transistor is small, so thatdeterioration of the characteristics of the transistor can be eased. Inspecific, since a period of time when the transistor 102 is on is long,the potential of the signal in the H level that is input to the wiring123C is preferably lower than V2. However, this embodiment is notlimited to this. The potential of the signal in the H level that isinput to the wiring 123A, the wiring 123B, and/or the wiring 123C can behigher than V2.

Note that the amplitude voltage of the signal that is input to thewiring 123A, the wiring 123B, and/or the wiring 123C can be lower than(V2−V1). In specific, since a period of time when the transistor 103 ison is long, the amplitude of the signal that is input to the wiring 123Ais preferably lower than (V2−V1). In this manner, since the Vgs of thetransistor 103 can be made small, deterioration of the characteristicsof the transistor 103 can be suppressed. However, this embodiment is notlimited to this. The amplitude voltage of the signal that is input tothe wiring 123A, the wiring 123B, and/or the wiring 123C can be higherthan (V2−V1).

Note that a signal can be input to the wiring 122A and/or the wiring122B. In this manner, since the voltage V1 can be eliminated, the numberof power supplies can be reduced. Alternatively, since backward bias canbe applied to the transistor, deterioration of the characteristics ofthe transistor can be eased. In specific, a signal whose level goes intoan L level during a period when the transistor 103 is on (e.g., theperiod T1, the period T3, and the period T5) can be input to the wiring122A. For example, an inverted signal (hereinafter also referred to asan inverted clock signal) of the signal S2 can be given. A signal whoselevel goes into an L level during a period when the transistor 104 is on(e.g., the period T3, the period T4, and the period T5) can be input tothe wiring 122B.

Note that voltage (e.g., the voltage V2) can be supplied to the wiring123A, the wiring 123B, and/or the wiring 123C. Accordingly, thesemiconductor device can function as an inverter circuit or a buffercircuit.

Note that as shown in FIG. 3A, since the same voltages (e.g., thevoltage V1) are supplied to the wiring 122A and 122B in many cases, thewiring 122A and the wiring 122B can be shared. Accordingly, the firstterminal of the transistor 103 and the first terminal of the transistor104 are connected to the wiring 122. The wiring 122 corresponds to thewiring 122A or the wiring 122B. A signal similar to that input to thewiring 122A or the wiring 122B can be input to the wiring 122.

Note that the terms “a plurality of wirings is shared” mean that anelement or a circuit which is connected to the plurality of wirings isconnected to one wiring. Alternatively, the terms “a plurality ofwirings is shared” mean that the plurality of wirings is connected toeach other.

Note that as shown in FIG. 3B, since the same signals (e.g., the signalS2) are input to the wirings 123A to 123C in many cases, the wiring 123Ato 123C can be shared. Accordingly, the first terminal of the transistor101, the gate of the transistor 102, and one electrode of the capacitor106 are connected to the wiring 123. The wiring 123 corresponds to thewirings 123A to 123C. A signal similar to that input to the wirings 123Ato 123C can be input to the wiring 123. However, this embodiment is notlimited to this. Any two or more of the wirings 123A to 123C can beshared.

Note that as in FIG. 3B, the wirings 123A to 123C can be shared in FIG.3A.

Note that as shown in FIG. 3C, by combining FIGS. 3A and 3B, the wiring122A and the wiring 122B can be shared and, further, the wirings 123A to123C can be shared. For example, the first terminal of the transistor103 and the first terminal of the transistor 104 can be connected to thewiring 122. In addition, the first terminal of the transistor 101, thegate of the transistor 102, and one electrode of the capacitor 106 canbe connected to the wiring 123.

Note that as shown in FIG. 3D, the gate of the transistor 104 can beconnected to the wiring 121. Since the gate of the transistor 104 isconnected to the wiring 121, the voltage of the gate when the transistor104 is turned on is V1 which is lower than the voltage (V1+Vth101+α) ofthe gate when the transistor 104 is turned on in FIG. 1A. Accordingly, adielectric breakdown of the transistor 104 or deterioration ofcharacteristics of the transistor 104 can be suppressed.

Note that as in FIG. 3D, the gate of the transistor 104 can be connectedto the wiring 121 in FIGS. 3A to 3C.

Note that as shown in FIG. 3E, the second terminal of the transistor 103can be connected to the wiring 121. Since the second terminal of thetransistor 103 is connected to the wiring 121, the voltage V1 issupplied from the wiring 122A to the wiring 121 during the period T4;therefore, the potential of the wiring 121 easily maintained as V1.

Note that as in FIG. 3E, the second terminal of the transistor 103 canbe connected to the wiring 121 in FIGS. 3A to 3D.

Note that as shown in FIG. 4A, the capacitor 105 can be eliminated. Inthat case, the parasitic capacitance between the gate and the secondterminal of the transistor 101 can be used as the capacitor 105.

Note that in FIG. 4A, in the case where the parasitic capacitancebetween the gate and the second terminal of the transistor 101 is usedas the capacitor 105, the parasitic capacitance between the gate and thesecond terminal of the transistor 101 is preferably higher than theparasitic capacitance between the gate and the first terminal of thetransistor 101. Accordingly, in the transistor 101, an area where aconductive layer which functions as a gate electrode and a conductivelayer which functions as a source electrode or drain electrode overlapwith each other on the second terminal side is preferably larger thanthat on the first terminal side. However, this embodiment is not limitedto this.

Note that as in FIG. 4A, the capacitor 105 can be eliminated in FIGS. 3Ato 3E.

Note that as shown in FIG. 4B, a MOS capacitor can be used as thecapacitor 105. In an example in FIG. 4B, a transistor 105 a is used asthe capacitor 105. The transistor 105 a is an n-channel transistor. Afirst terminal and a second terminal of the transistor 105 a areconnected to the wiring 121. A gate of the transistor 105 a is connectedto the node A. In this manner, since the potential of the node A is highduring a period in which the transistor 105 a needs to function as acapacitor (such a period is the periods T1 and T2), the gate capacitanceof the transistor 105 a can be high. On the other hand, since thepotential of the node A is low during a period in which the transistor105 a does not need to function as a capacitor (such a period is theperiods T3, T4, and T5), the gate capacitance of the transistor 105 acan be low. However, this embodiment is not limited to this. Thetransistor 105 a can be a p-channel transistor. Alternatively, one ofthe first terminal and the second terminal of the transistor 105 a canbe in a floating state. Alternatively, the gate of the transistor 105 acan be connected to the wiring 121. The first terminal and the secondterminal of the transistor 105 a can be connected to the node A.Alternatively, an impurity can be added to a channel region of thetransistor 105 a.

Note that as in FIG. 4B, the transistor 105 a can be used as thecapacitor 105, the first terminal and the second terminal of thetransistor 105 a can be connected to the wiring 121, and the gate of thetransistor 105 a can be connected to the node A in FIGS. 3A to 3E andFIG. 4A.

Note that as shown in FIG. 4C, a MOS capacitor can be used as thecapacitor 106. In an example in FIG. 4C, a transistor 106 a is used asthe capacitor 106. The transistor 106 a is an n-channel transistor. Afirst terminal and a second terminal of the transistor 106 a areconnected to the node B. A gate of the transistor 106 a is connected tothe wiring 123A. However, this embodiment is not limited to this. Thetransistor 106 a can be a p-channel transistor. Alternatively, one ofthe first terminal and the second terminal of the transistor 106 a canbe in a floating state. Alternatively, the gate of the transistor 106 acan be connected to the node B. The first terminal and the secondterminal of the transistor 106 a can be connected to the wiring 123A.Alternatively, an impurity can be added to a channel region of thetransistor 106 a.

Note that as in FIG. 4C, the transistor 106 a can be used as thecapacitor 106, the first terminal and the second terminal of thetransistor 106 a can be connected to the node B, and the gate of thetransistor 106 a can be connected to the wiring 123A in FIGS. 3A to 3Eand FIGS. 4A and 4B.

Note that as shown in FIG. 4D, the transistor 103 can be replaced with adiode 103 a. The diode 103 a corresponds to the transistor 103. Inaddition, the diode 103 a has a function of decreasing the potential ofthe node A when the potential of the node B is lower than that of thenode A, and a function of bringing the node A and the node B out ofelectrical conduction when the potential of the node B is higher thanthat of the node A. One terminal (hereinafter also referred to as aninput terminal or an anode) of the diode 103 a is connected to the nodeA. The other terminal (hereinafter also referred to as an outputterminal or a cathode) of the diode 103 a is connected to the node B.

Note that in the case where the transistor 103 is replaced with thediode 103 a in FIG. 4D, voltage V2 can be supplied to the wiring 122B.Alternatively, an inverted signal (e.g., an inverted clock signal) ofthe signal S2 can be input to the wiring 123A.

Note that as in FIG. 4D, the transistor 103 can be replaced with thediode 103 a, one terminal of the diode 103 a can be connected to thenode A, and the other terminal of the diode 103 a can be connected tothe node B in FIGS. 3A to 3E and FIGS. 4A to 4C.

Note that as shown in FIG. 4E, the transistor 104 can be replaced with adiode 104 a. In FIG. 4E, an example of the case where not only thetransistor 104 but also the transistor 103 is replaced with a diode isshown. The diode 104 a corresponds to the transistor 104. In addition,the diode 104 a has a function of raising the potential of the node Bwhen the potential of the node A is higher than that of the node B, anda function of bringing the node A and the node B out of electricalconduction when the potential of the node A is lower than that of thenode B. One terminal of the diode 104 a is connected to the node A. Theother terminal of the diode 104 a is connected to the node B.

Note that as in FIG. 4E, the transistor 104 can be replaced with thediode 104 a, one terminal of the diode 104 a can be connected to thenode A, and the other terminal of the diode 104 a can be connected tothe node B in FIGS. 3A to 3E and FIGS. 4A to 4D.

Note that as shown in FIG. 4F, a diode-connected transistor can be usedas a diode. The diode-connected transistor 103 and the diode-connectedtransistor 104 correspond to the diode 103 a and the diode 104 a,respectively. The first terminal of the transistor 103 is connected tothe node B. The second terminal and the gate of the transistor 103 areconnected to the node A. The first terminal and the gate of thetransistor 104 are connected to the node A. The second terminal of thetransistor 104 is connected to the node B. However, this embodiment isnot limited to this. The gate of the transistor 103 can be connected tothe node B and the gate of the transistor 104 can be connected to thenode B.

Note that as in FIG. 4F, the first terminal of the transistor 103 can beconnected to the node B, the second terminal of the transistor 103 canbe connected to the node A, and the gate of the transistor 103 can beconnected to the node A in FIGS. 3A to 3E and FIGS. 4A to 4E.Alternatively, the first terminal of the transistor 104 can be connectedto the node A, the second terminal of the transistor 104 can beconnected to the node B, and the gate of the transistor 104 can beconnected to the node A. However, this embodiment is not limited tothis. The gate of the transistor 103 can be connected to the node B andthe gate of the transistor 104 can be connected to the node B.

Note that as shown in FIG. 5A, a diode 107 can be additionally provided.The diode 107 has a function of decreasing the potential of the node Bwhen a signal in an L level is input to the wiring 123A and a functionof bringing the wiring 123A and the node B out of electrical conductionwhen a signal in an H level is input to the wiring 123A. One terminal ofthe diode 107 is connected to the node B. The other terminal of thediode 107 is connected to the wiring 123A. However, this embodiment isnot limited to this. The other terminal of the diode 107 can beconnected to a different wiring from the wiring 123A.

Note that as in FIG. 5A, the diode 107 can be additionally provided, oneterminal of the diode 107 can be connected to the node B, and the otherterminal of the diode 107 can be connected to the wiring 123A in FIGS.3A to 3E and FIGS. 4A to 4F.

Note that as shown in FIG. 5B, a diode-connected transistor 107 a can beadditionally provided. The diode-connected transistor 107 a correspondsto the diode 107 and is an n-channel transistor. A first terminal of thetransistor 107 a is connected to the wiring 123A. A second terminal anda gate of the transistor 107 a are connected to the node B. However,this embodiment is not limited to this. The transistor 107 a can be ap-channel transistor. Alternatively, the gate of the transistor 107 acan be connected to the wiring 123A.

Note that as in FIG. 5B, the transistor 107 a can be additionallyprovided, the first terminal of the transistor 107 a can be connected tothe wiring 123A, and the second terminal and the gate of the transistor107 a can be connected to the node B in FIGS. 3A to 3E, FIGS. 4A to 4F,and FIG. 5A. However, this embodiment is not limited to this. The gateof the transistor 107 a can be connected to the node B.

Note that as shown in FIG. 5C, the transistor 102 can be eliminated.

Note that as in FIG. 5C, the transistor 102 can be eliminated in FIGS.3A to 3E, FIGS. 4A to 4F, and FIGS. 5A and 5B.

Note that as shown in FIG. 5D, the circuit 100 can be eliminated.

Note that as in FIG. 5D, the circuit 100 can be omitted in FIGS. 3A to3E, FIGS. 4A to 4F, and FIGS. 5A to 5C.

Note that as shown in FIG. 5E, the transistor 101, the transistor 102,the transistor 103, and the transistor 104 can be replaced with atransistor 101 p, a transistor 102 p, a transistor 103 p, and atransistor 104 p, respectively. The transistors 101 p to 104 pcorrespond to the transistors 101 to 104, respectively, and arep-channel transistors.

Note that in FIG. 5E, a relation of potential is opposite to that in thesemiconductor device in FIG. 1A in many cases. For example, the voltageV2 can be supplied to the wirings 122A and 122B and an inverted signalof the signal S2 can be input to the wirings 123A and 123B. Similarly,an inverted signal of the signal S1 is output from the wiring 121 inmany cases.

Note that in FIG. 5E, the circuit 100 has a function of decreasing thepotential of the node A during the period T1 in many cases.Alternatively, the circuit 100 has a function of raising the potentialof the node A to V2 during the period T3 in many cases.

Note that as in FIG. 5E, p-channel transistors can be used as thetransistors 101 to 104 in FIGS. 3A to 3E, FIGS. 4A to 4F, and FIGS. 5Ato 5D.

Embodiment 2

In this embodiment, one example of the semiconductor device will bedescribed. The semiconductor device of this embodiment is a specificexample of the semiconductor device described in Embodiment 1. Inspecific, a specific example of the circuit 100 will be described inthis embodiment. Note that the content described in Embodiment 1 can beapplied to the semiconductor device in this embodiment.

The specific example of the circuit 100 will be explained with referenceto FIG. 6A. However, FIG. 6A is one example and this embodiment is notlimited to this. Circuits of a variety of structures can be used as thecircuit 100 besides the circuit with the structure shown in FIG. 6A.Note that a portion which is similar to that in FIG. 1A is denoted bythe same reference numeral and the description thereof is omitted.

The circuit 100 includes a transistor 131, a transistor 132, atransistor 133, a transistor 134, and a transistor 135. Each of thetransistors 131 to 135 is an n-channel transistor. However, each of thetransistors 131 to 135 can be a p-channel transistor.

A connection relation of the transistors included in the circuit 100will be described. A first terminal of the transistor 131 is connectedto a wiring 125. A second terminal of the transistor 131 is connected tothe node A. A gate of the transistor 131 is connected to the wiring 125.A first terminal of the transistor 132 is connected to the wiring 125. Asecond terminal of the transistor 132 is connected to the node A. A gateof the transistor 132 is connected to the wiring 124A. A first terminalof the transistor 133 is connected to a wiring 122E. A second terminalof the transistor 133 is connected to the wiring 121. A gate of thetransistor 133 is connected to a wiring 124B. A first terminal of thetransistor 134 is connected to a wiring 122C. A second terminal of thetransistor 134 is connected to the node A. A gate of the transistor 134is connected to the wiring 126. A first terminal of the transistor 135is connected to a wiring 122D. A second terminal of the transistor 135is connected to the wiring 121. A gate of the transistor 135 isconnected to the wiring 126.

An example of a thing (e.g., a signal, voltage, or current) which can beinput to the wirings 122C to 122E, the wirings 124A and 124B, the wiring125, and the wiring 126 will be described. However, the contentdescribed below is one example and this embodiment is not limited tothis. A variety of things besides that described below can be input toeach wiring. In addition, each wiring can be made in a floating state.

Like the wirings 122A and 122B, the voltage V1 is supplied to thewirings 122C to 122E. Accordingly, the wirings 122C to 122E can functionas power supply lines. However, this embodiment is not limited to this.A signal such as a clock signal can be input to the wirings 122C to122E. In that case, the wirings 122C to 122E can function as signallines. Alternatively, different voltages can be supplied to the wirings122C to 122E.

For example, a signal S3 is input to the wirings 124A and 124B.Accordingly, the wirings 124A and 124B can function as signal lines. Thesignal S3 is an inverted signal of the signal S2 or a signal which isout of phase with the signal S2 by approximately 180° in many cases andcan function as an inverted clock signal (CKB). However, this embodimentis not limited to this. Voltage can be supplied to the wirings 124A and124B. In that case, the wirings 124A and 124B can function as powersupply lines. Alternatively, different signals can be input to thewirings 124A and 124B.

For example, a signal S4 is input to the wiring 125. Accordingly, thewiring 125 can function as a signal line. The signal S4 is a digitalsignal with an L level and an H level in many cases and can function asa start signal (SP), a transfer signal from a different row (stage), ora signal for selecting a different row. However, this embodiment is notlimited to this. Voltage can be supplied to the wiring 125. In thatcase, the wiring 125 can function as a power supply line.

For example, a signal S5 is input to the wiring 126. Accordingly, thewiring 126 can function as a signal line. The signal S5 is a digitalsignal with an L level or an H level in many cases and can function as areset signal (RE) or a signal for selecting a different row. However,this embodiment is not limited to this. Voltage can be supplied to thewiring 126. In that case, the wiring 126 can function as a power supplyline.

One example of functions of the transistors 131 to 135 will bedescribed. However, the content described below is one example and thisembodiment is not limited to this. The transistors 131 to 135 can have avariety of functions besides that described below. Alternatively, it ispossible that the transistors 131 to 135 do not have the functionsdescribed below.

The transistor 131 has a function of raising the potential of the node Ain accordance with a signal (e.g., the signal S4) input to the wiring125 and functions as a diode. The transistor 132 has a function ofcontrolling a timing when the wiring 125 and the node A are brought intoelectrical conduction in accordance with a signal (e.g., the signal S3)input to the wiring 124A and functions as a switch. The transistor 133has a function of controlling a timing when the wiring 122E and thewiring 121 are brought into electrical conduction in accordance with asignal (e.g., the signal S3) input to the wiring 124B and functions as aswitch. The transistor 134 has a function of controlling a timing whenthe wiring 122C and the node A are brought into electrical conduction inaccordance with a signal (e.g., the signal S5) input to the wiring 126and functions as a switch. The transistor 135 has a function ofcontrolling a timing when the wiring 122D and the wiring 121 are broughtinto electrical conduction in accordance with a signal (e.g., the signalS5) input to the wiring 126 and functions as a switch.

Next, operation of the semiconductor device in FIG. 6A will be describedwith reference to FIG. 6B, FIGS. 7A to 7C, and FIGS. 8A and 8B. FIG. 6Bis one example of a timing chart for illustrating the operation of thesemiconductor device and there are a period T1, a period T2, a periodT3, a period T4, and a period T5. FIG. 7A is a schematic view of theoperation of the semiconductor device in FIG. 6A during the period T1.FIG. 7B is a schematic view of the operation of the semiconductor devicein FIG. 6A during the period T2. FIG. 7C is a schematic view of theoperation of the semiconductor device in FIG. 6A during the period T3.FIG. 8A is a schematic view of the operation of the semiconductor devicein FIG. 6A during the period T4. FIG. 8B is a schematic view of theoperation of the semiconductor device in FIG. 6A during the period T5.Note that description of operation in common with the semiconductordevice in FIG. 1A is omitted.

First, during the period T1, since the signal S5 is in an L level, thetransistor 134 and the transistor 135 are turned off. Accordingly, thewiring 122C and the node A are brought out of electrical conduction andthe wiring 122D and the wiring 121 are brought out of electricalconduction. At the same time, since the signal S3 and the signal S4 aremade to be in an H level, the transistor 131, the transistor 132, andthe transistor 133 are turned on. Then, the wiring 125 and the node Aare brought into electrical conduction and the wiring 122E and thewiring 121 are brought into electrical conduction. Accordingly, thesignal (the signal S4 in the H level) input to the wiring 125 issupplied from the wiring 125 to the node A, whereby the potential of thenode A starts to be raised. Further, since the wiring 122E and thewiring 121 are brought into electrical conduction, the voltage V1 issupplied from the wiring 122E to the wiring 121. After that, when thepotential of the node A is raised to a value obtained by subtracting thethreshold voltage (Vth131) of the transistor 131 from the potential (V2)in the H level of the signal S4, (V2−Vth131), the transistor 131 isturned off. Similarly, when the potential of the node A is raised to avalue obtained by subtracting the threshold voltage (Vth132) of thetransistor 132 from the potential (V2) in the H level of the signal S3,(V2−Vth132), the transistor 132 is turned off. When the transistor 131and the transistor 132 are off, charge is not supplied to the node A.Accordingly, the potential of the node A is maintained as a large value(at least greater than or equal to V1+Vth101) and the node A goes into afloating state. Here, for simplicity, the transistor 131 and thetransistor 132 are turned off when the potential of the node A becomes(V2−Vth131). Accordingly, the wiring 125 and the node A are brought outof electrical conduction. The potential of the node A at that time ismaintained as (V1−Vth131) and the node A goes into a floating state.

Next, during the period T2, since the signal S4 is in an L level, thetransistor 131 is kept off. Then, since the signal S3 goes into an Llevel, the transistor 132 is kept off and the transistor 133 is turnedoff. Accordingly, the wiring 125 and the node A are kept out ofelectrical conduction and the wiring 122E and the wiring 121 are broughtout of electrical conduction. At that time since the signal S5 is keptin the L level, the transistor 134 and the transistor 135 are kept off.Accordingly, the wiring 122C and the node A are kept out of electricalconduction and the wiring 122D and the wiring 121 are kept out ofelectrical conduction.

Next, during the period T3, since the signal S4 is kept in the L level,the transistor 131 is kept off. Then, since the signal S5 goes into an Hlevel, the transistor 134 and the transistor 135 are turned on.Accordingly, the wiring 122C and the node A are brought into electricalconduction and the wiring 122D and the wiring 121 are brought intoelectrical conduction. Accordingly, since the voltage V1 is suppliedfrom the wiring 122C to the node A, the potential of the node A isdecreased to V1. Similarly, since the voltage V1 is supplied from thewiring 122D to the wiring 121, the potential of the wiring 121 isdecreased to V1. At the same time, since the signal S3 goes into an Hlevel, the transistor 132 and the transistor 133 are turned on. Thus,the wiring 125 and the node A are brought into electrical conduction andthe wiring 122E and the wiring 121 are brought into electricalconduction. Accordingly, since the signal S4 in the L level is suppliedto the node A, the potential of the node A is decreased to V1.Similarly, since the voltage V1 is supplied to the wiring 121, thepotential of the wiring 121 is decreased to V1.

Next, during the period T4, since the signal S4 is kept in the L level,the transistor 131 is kept off. Then, since the signal S5 goes into an Llevel, the transistor 134 and the transistor 135 are turned off.Accordingly, the wiring 122C and the node A are brought out ofelectrical conduction and the wiring 122D and the wiring 121 are broughtout of electrical conduction. At that time, since the signal S4 goesinto an L level, the transistor 132 and the transistor 133 are turnedoff. Accordingly, the wiring 125 and the node A are brought out ofelectrical conduction and the wiring 122E and the wiring 121 are broughtout of electrical conduction.

Next, during the period T5, since the signal S4 is kept in the L level,the transistor 131 is kept off. Then, since the signal S5 is kept in theL level, the transistor 134 and the transistor 135 are kept off.Accordingly, the wiring 122C and the node A are kept out of electricalconduction and the wiring 122D and th wiring 121 are kept out ofelectrical conduction. At that time, since the signal S3 goes into an Hlevel, the transistor 132 and the transistor 133 are turned on.Accordingly, the wiring 125 and the node A are brought into electricalconduction and the wiring 122E and the wiring 121 are brought intoelectrical conduction. Accordingly, since the signal S4 in the L levelis supplied from the wiring 125 to the node A, the potential of the nodeA is maintained as V1. Similarly, since the voltage V1 is supplied fromthe wiring 122E to the wiring 121, the potential of the wiring 121 ismaintained as V1.

In the semiconductor device in FIG. 6A, since the signal in the L levelor the voltage V1 is supplied to the node A during the periods T4 andT5, noise of the node A can be reduced. Therefore, malfunction can beprevented.

Alternatively, in the semiconductor device in FIG. 6A, since both of thetransistor 131 and the transistor 132 are turned on during the periodT1, the potential of the node A can be quickly raised. Alternatively,the channel width of the transistor 131 or the channel width of thetransistor 132 can be made small.

Note that the channel width of the transistor 131 can be larger thanthat of the transistor 134 or the transistor 103. Similarly, the channelwidth of the transistor 132 can be larger than that of the transistor134 or the transistor 103. This is because the potential of the node Ais preferably raised quickly during the period T2 and the potential ofthe node A is preferably decreased slowly during the period T3. That is,when the potential of the node A is raised quickly during the period T2,increase in driving frequency, suppression of through current, reductionin power consumption, or the like can be achieved. On the other hand,when the potential of the node A is decreased slowly during the periodT3, an on time of the transistor 101 becomes long, whereby a rising timeof a signal (e.g., the signal S1) output from the wiring 121 can beshortened. Therefore, the channel width of the transistor that has afunction of raising the potential of the node A during the period T2 ispreferably larger than that of the transistor that decreases thepotential of the node A during the period T3. However, this embodimentis not limited to this. The channel width of the transistor 131 can besmaller than that of the transistor 134 or the transistor 103.Similarly, the channel width of the transistor 132 can be smaller thanthat of the transistor 134 or the transistor 103.

Note that the sum of the channel width of the transistor 131 and thechannel width of the transistor 134 can be larger than the channel widthof the transistor 134 or the channel width of the transistor 103. Thisis because, during the period T2, the signal S4 in the H level issupplied from the wiring 125 to the node A through two transistors ofthe transistor 131 and the transistor 132 connected in parallel.However, this embodiment is not limited to this. The sum of the channelwidth of the transistor 131 and the channel width of the transistor 134can be smaller than the channel width of the transistor 134 or thechannel width of the transistor 103.

Note that the channel width of the transistor 134 can be smaller thanthat of the transistor 133. Similarly, the channel width of thetransistor 132 can be smaller than that of the transistor 133.Similarly, the channel width of the transistor 103 can be smaller thanthat of the transistor 102. This is because the load (e.g., wiringresistance, parasitic capacitance, a transistor to be connected, or thelike) of the wiring 121 is higher than that of the node A in many cases.Accordingly, the channel width of a transistor that has a function ofsupplying a signal or voltage to the node A is preferably smaller thanthat of a transistor that supplies a signal or voltage to the wiring121. However, this embodiment is not limited to this. The channel widthof the transistor 134 can be larger than that of the transistor 133.Similarly, the channel width of the transistor 132 can be larger thanthat of the transistor 133. Similarly, the channel width of thetransistor 103 can be larger than that of the transistor 102.

Note that the channel width of the transistor 103 can be larger thanthat of the transistor 132. This is because the transistor 103 has afunction of maintaining the potential of the node A as V1 during theperiod T4 while the transistor 132 has a function of maintaining thepotential of the node A as V1 during the period T5. In specific, asignal (e.g., the signal S2) input to the wiring 123B is in an H levelduring the period T4. At that time, if the potential of the node A israised and the transistor 101 is turned on, the potential of the wiring121 is raised. Therefore, since the transistor 103 is required tomaintain the potential of the node A as V1 and keep the transistor 101off, the channel width of the transistor 103 is preferably large. On theother hand, since a signal (e.g., the signal S2) input to the wiring123B is in an L level during the period T5, the potential of the wiring121 is not raised even if the transistor 101 is turned on. That is, evenif the potential of the node A is raised or decreased from V1, thepotential of the wiring 121 is not raised. Therefore, since there is nogreat necessity for reducing the on resistance of the transistor 132,the channel width of the transistor 132 is preferably small. However,this embodiment is not limited to this. The channel width of thetransistor 103 can be smaller than that of the transistor 132. This isbecause the transistor 132 has a function of raising the potential ofthe node A during the period T1. By increasing the channel width of thetransistor 132, the potential of the node A can be quickly raised.

Note that the channel width of the transistor 102 can be smaller thanthat of the channel width of the transistor 133. This is because, if thechannel width of the transistor 102 is too large, the potential of thenode A decreases too much during the period T2, whereby thesemiconductor device malfunctions. In specific, both of the transistor102 and the transistor 133 have a function of maintaining the potentialof the wiring 121 as V1. However, during the period T2, the transistor102 is on until the potential of the wiring 121 is raised to a valueobtained by subtracting the threshold voltage (Vth102) of the transistor102 from the potential (V2) of the wiring 123C, (V2−Vth102). Therefore,in order to prevent the potential of the node A from decreasing too muchduring the period T2, the channel width of the transistor 102 ispreferably small. On the other hand, the channel width of the transistor133 is preferably large in order to maintain the potential of the wiring121 as V1. However, this embodiment is not limited to this. The channelwidth of the transistor 102 can be larger than that of the transistor133. This is because there is a high possibility that the potential ofthe wiring 121 is raised when the signal S2 goes into an H level duringthe period T4. Therefore, by increasing the channel width of thetransistor 102, rise of the potential of the wiring 121 can be easilysuppressed.

Note that as in Embodiment 1, the potential of the signal in the L levelwhich is input to the wiring 124A, the wiring 124B, the wiring 125,and/or the wiring 126 can be lower than V1. In specific, since a periodof time when the transistor 132 and the transistor 133 are on is long,the potential of the signal in the L level which is input to the wiring124A and the wiring 124B is preferably lower than V1.

Note that as in Embodiment 1, the potential of the signal in the H levelwhich is input to the wiring 124A, the wiring 124B, the wiring 125,and/or the wiring 126 can be lower than V2. In specific, since thetransistor 132 and the transistor 133 easily deteriorate, the potentialof the signal in the H level which is input to the wiring 124A and thewiring 124B is preferably lower than V2.

Note that as in Embodiment 1, a signal can be input to the wiring 122C,the wiring 122D, or the wiring 122E. For example, a signal which goesinto an L level during a period (e.g., the period T3) in which thetransistor 134 is on can be input to the wiring 122C. For example, thesignal S2 or the signal S4 can be given as the signal. A signal whichgoes into an L level during a period (e.g., the period T3) in which thetransistor 135 is on can be input to the wiring 122D. For example, thesignal S2 or the signal S4 can be given as the signal. A signal whichgoes into an L level during a period (e.g., the period T1, the periodT3, and the period T5) in which the transistor 133 is on can be input tothe wiring 122E. For example, the signal S2 or the signal S3 can begiven as the signal.

Note that FIG. 13C shows a structure in which the first terminal of thetransistor 103 is connected to the wiring 124B, the first terminal ofthe transistor 104 is connected to the wiring 126, the first terminal ofthe transistor 133 is connected to the wiring 123A, the first terminalof the transistor 134 is connected to the wiring 123A, and the firstterminal of the transistor 135 is connected to the wiring 123A, forexample. However, this embodiment is not limited to this. The firstterminal of the transistor 103 can be connected to the wiring 124A orthe wiring 125. Alternatively, the first terminal of the transistor 133,the first terminal of the transistor 134, or the first terminal of thetransistor 135 can be connected to the wiring 121, the wiring 123B, thewiring 123C, or the wiring 126.

Note that as in Embodiment 1, voltage (e.g., the voltage V1 or thevoltage V2) can be supplied to the wiring 124A, the wiring 124B and/orthe wiring 126. In this manner, the semiconductor device can function asan inverter circuit or a buffer circuit.

Note that as shown in FIG. 9A, since the same signal (e.g., the signalS3) is input to the wiring 124A and the wiring 124B, the wiring 124A andthe wiring 124B can be shared. Accordingly, the gate of the transistor132 and the gate of the transistor 133 are connected to the wiring 124.The wiring 124 corresponds to the wiring 124A or the wiring 124B. Asignal similar to that input to these wirings can be input to the wiring124.

Note that FIG. 9C shows a structure in which FIG. 3C and FIG. 9A arecombined. For example, the first terminal of the transistor 101, thegate of the transistor 102, and one electrode of the capacitor 106 areconnected to the wiring 123. The gate of the transistor 132 and the gateof the transistor 133 are connected to the wiring 124. The firstterminal of the transistor 103, the first terminal of the transistor104, the first terminal of the transistor 133, the first terminal of thetransistor 134, and the first terminal of the transistor 135 areconnected to the wiring 122.

Note that as shown in FIG. 9C, the gate of the transistor 131 can beconnected to the wiring 127. For example, the voltage V2 is supplied tothe wiring 127 and the wiring 127 can function as a power supply line.However, this embodiment is not limited to this. A variety of thingssuch as current, voltage, or a signal can be input to the wiring 127.For example, since a signal that is input to the wiring 127 ispreferably in an H level during the period T1 and is in an L levelduring the period T2, the signal S3 can be input to the wiring 127. Inthat case, the wiring 127 can be connected to the wiring 124A or thewiring 124B and function as a signal line.

Note that in FIG. 9C, although the gate of the transistor 131 isconnected to the wiring 127, this embodiment is not limited to this. Forexample, the first terminal of the transistor 131 can be connected tothe wiring 127 and the gate of the transistor 131 can be connected tothe wiring 125.

Note that as in FIG. 9C, the gate of the transistor 131 can be connectedto the wiring 127 in FIGS. 9A and 9B.

Note that as shown in FIG. 10A, the transistor 131 can be eliminated.Even though the transistor 131 is eliminated, the potential of the nodeA is raised because the transistor 132 is on during the period T1.

Note that as in FIG. 10A, the transistor 131 can be eliminated in FIGS.9A to 9C.

Note that as shown in FIG. 10B, the transistor 132 can be eliminated.Even though the transistor 132 is eliminated, the potential of the nodeA is maintained as V1 because the node A goes into a floating state inthe period T5.

Note that as in FIG. 10B, the transistor 132 can be eliminated in FIGS.9A to 9C and FIG. 10A.

Note that as shown in FIG. 10C, the transistor 134 and the transistor135 can be eliminated. Alternatively, one of the transistor 134 and thetransistor 135 can be eliminated. Even though the transistor 134 iseliminated, the potential of the node A is decreased to V1 because thetransistor 132 is turned on in the period T3. Similarly, even though thetransistor 135 is eliminated, the potential of the wiring 121 isdecreased to V1 because the transistor 133 is turned on in the periodT3.

Note that as in FIG. 10C, the transistor 134 and the transistor 135 canbe eliminated in FIGS. 9A to 9C and FIGS. 10A and 10B.

Note that as shown in FIG. 11A, the transistor 133 can be eliminated.Even though the transistor 133 is eliminated, the potential of thewiring 121 is maintained as V1 because the wiring 121 goes into afloating state in the period T5.

Note that as in FIG. 11A, the transistor 133 can be eliminated in FIGS.9A to 9C and FIGS. 10A to 10C.

Note that as shown in FIG. 11B, the transistor 102 can be eliminated.Even though the transistor 102 is eliminated, the potential of thewiring 121 is maintained as V1 because the wiring 121 goes into afloating state in the period T4.

Note that as in FIG. 11B, the transistor 102 can be eliminated in FIGS.9A to 9C, FIGS. 10A to 10C, and FIG. 11A.

Note that as shown in FIG. 11C, the transistor 103, the transistor 104,and the capacitor 106 can be eliminated. Even though the transistor 103,the transistor 104, and the capacitor 106 are eliminated, the potentialof the wiring 121 is maintained as V1 because the wiring 121 goes into afloating state in the period T4.

Note that as in FIG. 11C, the transistor 103, the transistor 104, andthe capacitor 106 can be eliminated in FIGS. 9A to 9C, FIGS. 10A to 10C,and FIGS. 11A and 11B.

Note that as shown in FIG. 12A, the transistor 133 can be replaced witha diode 133 a. The diode 133 a corresponds to the transistor 133. Thediode 133 a has a function of decreasing the potential of the wiring 121when a signal in an L level is input to the wiring 124B, and a functionof bringing the wiring 124B and the wiring 121 out of electricalconduction when a signal in an H level is input to the wiring 124B. Oneterminal (hereinafter referred to as an input terminal or an anode) ofthe diode 133 a is connected to the wiring 121. The other terminal(hereinafter referred to as an output terminal or a cathode) of thediode 133 a is connected to the wiring 124B.

Note that in FIG. 12A, in the case where the transistor 133 is replacedwith the diode 133 a, the signal S2 can be input to the wiring 124B.Therefore, the wiring 124B can be connected to the wirings 123A to 123Cand the wiring 124B and the wirings 123A to 123C can be shared.

Note that as in FIG. 12A, the transistor 133 can be replaced with thediode 133 a, one terminal of the diode 133 a can be connected to thewiring 121, and the other terminal of the diode 133 a can be connectedto the wiring 124B in FIGS. 9A to 9C, FIGS. 10A to 10C, and FIGS. 11A to11C.

Note that as shown in FIG. 12B, the transistor 133 can bediode-connected. The diode-connected transistor 133 corresponds to thediode 133 a. The first terminal of the transistor 133 is connected tothe wiring 124B. The second terminal of the transistor 133 is connectedto the wiring 121. The gate of the transistor 133 is connected to thewiring 121. However, this embodiment is not limited to this. The gate ofthe transistor 133 can be connected to the wiring 124B.

Note that as in FIG. 12B, the first terminal of the transistor 133 canbe connected to the wiring 124B, the second terminal of the transistor133 can be connected to the wiring 121, and the gate of the transistor133 can be connected to the wiring 121 in FIGS. 9A to 9C, FIGS. 10A to10C, FIGS. 11A to 11C, and FIG. 12A. However, this embodiment is notlimited to this. The gate of the transistor 133 can be connected to thewiring 124B.

Note that as shown in FIG. 12C, the transistor 134 can be replaced witha diode 134 a, and the transistor 135 can be replaced with a diode 135a. The diode 134 a and the diode 135 a correspond to the transistor 134and the transistor 135, respectively. The diode 134 a has a function ofdecreasing the potential of the node A when a signal in an L level isinput to the wiring 126, and a function of bringing the wiring 126 andthe node A out of electrical conduction when a signal in an H level isinput to the wiring 126. The diode 135 a has a function of decreasingthe potential of the wiring 121 when a signal in an L level is input tothe wiring 126, and a function of bringing the wiring 126 and the wiring121 out of electrical conduction when a signal in an H level is input tothe wiring 126. One terminal (hereinafter referred to as an inputterminal or an anode) of the diode 134 a is connected to the node A. Theother terminal (hereinafter referred to as an output terminal or acathode) of the diode 134 a is connected to the wiring 126. One terminal(hereinafter referred to as an input terminal or an anode) of the diode135 a is connected to the wiring 121. The other terminal (hereinafterreferred to as an output terminal or a cathode) of the diode 135 a isconnected to the wiring 126.

Note that in the case where the transistor 134 and the transistor 135are replaced with diodes in FIG. 12C, an inverted signal of the signalS5 can be input to the wiring 126, for example.

Note that in FIG. 12C, only one of the transistor 134 and the transistor135 can be replaced with a diode.

Note that as in FIG. 12C, the transistor 134 can be replaced with thediode 134 a, one terminal of the diode 134 a can be connected to thenode A, and the other terminal of the diode 134 a can be connected tothe wiring 126 in FIGS. 9A to 9C, FIGS. 10A to 10C, FIGS. 11A to 11C,and FIGS. 12A and 12B. Alternatively, the transistor 135 can be replacedwith the diode 135 a, one terminal of the diode 135 a can be connectedto the wiring 121, and the other terminal of the diode 135 a can beconnected to the wiring 126.

Note that as shown in FIG. 13A, the transistor 134 and the transistor135 can be diode-connected. The diode-connected transistor 134 and thediode-connected transistor 135 correspond to the diode 134 a and thediode 135 a, respectively. The first terminal of the transistor 134 isconnected to the wiring 126. The second terminal of the transistor 134is connected to the node A. The gate of the transistor 134 is connectedto the node A. The first terminal of the transistor 135 is connected tothe wiring 126. The second terminal of the transistor 135 is connectedto the wiring 121. The gate of the transistor 135 is connected to thewiring 121. However, this embodiment is not limited to this. The gate ofthe transistor 134 can be connected to the wiring 126. The gate of thetransistor 135 can be connected to the wiring 126.

Note that as in FIG. 13A, the first terminal of the transistor 134 canbe connected to the wiring 126, the second terminal of the transistor134 can be connected to the node A, and the gate of the transistor 134can be connected to the node A in FIGS. 9A to 9C, FIGS. 10A to 10C,FIGS. 11A to 11C, and FIGS. 12A to 12C. Alternatively, the firstterminal of the transistor 135 can be connected to the wiring 126, thesecond terminal of the transistor 135 can be connected to the wiring121, and the gate of the transistor 135 can be connected to the wiring121. However, this embodiment is not limited to this. The gate of thetransistor 134 can be connected to the wiring 126. The gate of thetransistor 135 can be connected to the wiring 126.

Note that as shown in FIG. 13B, a transistor 137 and a transistor 138can be additionally provided. The transistor 137 and the transistor 138are n-channel transistors. However, this embodiment is not limited tothis. The transistor 137 and the transistor 138 can be p-channeltransistors. A first terminal of the transistor 137 is connected to thewiring 122F. A second terminal of the transistor 137 is connected to thewiring 121. A gate of the transistor 137 is connected to the wiring 128.A first terminal of the transistor 138 is connected to the wiring 122G Asecond terminal of the transistor 138 is connected to the node A. A gateof the transistor 138 is connected to the wiring 128. For example, asignal S6 is input to the wiring 128. Therefore, the wiring 128 canfunction as a signal line. The signal S6 is a digital signal with an Hlevel and an L level in many cases. For example, the signal S6 canfunction as a signal which resets all the stages. For example, thevoltage V1 is supplied to the wiring 122F and the wiring 122G Therefore,the wiring 122F and the wiring 122G can function as power supply lines.Accordingly, the wirings 122A to 122G can be shared. In that case, thefirst terminal of the transistor 137 and the first terminal of thetransistor 138 can be connected to the wiring 122 as shown in FIG. 11B.However, a variety of things such as current, voltage, or signal can beinput to the wiring 128, the wiring 122F, and the wiring 122G.

Note that in FIG. 13B, the signal S6 can be in an H level during aperiod before a semiconductor device starts to operate. Alternatively,in the case where a semiconductor device shown in FIG. 13B is used as ashift register, the signal S6 can be in an H level during a periodbefore the shift register starts to scan or a period after the shiftregister has completed the scanning. Therefore, as the signal S6, astart pulse of the shift register, an output signal from the loweststage of the shift register, or the like can be used. However, oneexample of this embodiment is not limited to this.

Note that in FIG. 13B, only one of the transistor 137 and the transistor138 can be additionally provided.

Note that as in FIG. 13B, the transistor 137 can be additionallyprovided, the first terminal of the transistor 137 can be connected tothe wiring 122F, the second terminal of the transistor 137 can beconnected to the wiring 121, and the gate of the transistor 137 can beconnected to the wiring 128 in FIGS. 9A to 9C. FIGS. 10A to 10C, FIGS.11A to 11C, FIGS. 12A to 12C, and FIG. 13A. Alternatively, thetransistor 138 can be additionally provided, the first terminal of thetransistor 138 can be connected to the wiring 122G, the second terminalof the transistor 138 can be connected to the node A, and the gate ofthe transistor 138 can be connected to the wiring 128.

Embodiment 3

In this embodiment, one example of a shift register is described. Theshift register in this embodiment can include the semiconductor deviceof Embodiment 1 and Embodiment 2. Note that the shift register can alsobe referred to as a semiconductor device or a gate driver. Note that thecontent described in Embodiment 1 and Embodiment 2 can be applied tothat of the shift register in this embodiment.

First, one example of the shift register will be described withreference to FIG. 14A. A shift register 220 is connected to wirings201_1 to 201_N (N is a natural number), a wiring 202, a wiring 203, awiring 204, a wiring 205, and a wiring 206.

The wiring 202 corresponds to the wiring 123 (the wirings 123A to 123C)described in Embodiment 1 and Embodiment 2 or the wiring 124 (thewirings 124A and 124B) described in Embodiment 1 and Embodiment 2 andcan function as a signal line or a clock signal line. In addition, asignal GS2 is input from a circuit 221 to the wiring 202. The signal GS2corresponds to the signal S2 or the signal S3 described in Embodiment 1and Embodiment 2 and can function as a clock signal.

The wiring 203 corresponds to the wiring 123 (the wirings 123A to 123C)described in Embodiment 1 and Embodiment 2 or the wiring 124 (thewirings 124A and 124B) described in Embodiment 1 and Embodiment 2 andcan function as a signal line or a clock signal line. In addition, asignal GS3 is input from the circuit 221 to the wiring 203. The signalGS3 corresponds to the signal S2 or the signal S3 described inEmbodiment 1 and Embodiment 2 and can function as an inverted clocksignal.

The wiring 204 corresponds to the wiring 122 (the wirings 122A to 122G)described in Embodiment 1 and Embodiment 2 and can function as a powersupply line. In addition, voltage V1 is input from the circuit 221 tothe wiring 204.

The wiring 205 corresponds to the wiring 125 described in Embodiment 1and Embodiment 2 and can function as a signal line. In addition, asignal GS4 is input from the circuit 221 to the wiring 205. The signalGS4 corresponds to the signal S4 described in Embodiment 1 andEmbodiment 2 and can function as a start signal (hereinafter referred toas a start pulse) or a vertical synchronizing signal.

The wiring 206 corresponds to the wiring 126 described in Embodiment 1and Embodiment 2 and can function as a signal line. In addition, asignal GS5 is input from a circuit 221 to the wiring 206. The signal GS5corresponds to the signal S5 described in Embodiment 1 and Embodiment 2and can function as a reset signal.

However, this embodiment is not limited to the above description. Avariety of things such as a signal, voltage, or current can be input tothe wirings 202 to 206. Each wiring can be in a floating state.

Note that as shown in FIG. 6C, an unbalanced clock signal can be used asthe signal S2 or the signal S3. In that case, for example, the signal S3can have a phase which is deviated from that of the S2 by 180°.Accordingly, in the case where the semiconductor device of thisembodiment is used as a shift register, a selection signal in one stagecan be prevented from overlapping with a selection signal in theprevious stage or the next stage.

The wirings 201_1 to 201_N each correspond to the wiring 121 describedin Embodiment 1 and Embodiment 2 and can each function as a gate line ora scan line. In addition, signals GS1_1 to GS1_N are output from thewirings 201_1 to 201_N, respectively. The signals GS1_1 to GS1_N eachcorrespond to the signal S1 described in Embodiment 1 and Embodiment 2and can each function as an output signal, a selection signal, ascanning signal, or a gate signal.

Note that as shown in FIG. 14B, the signals GS1_1 to GS1_N go into an Hlevel in order from the signal GS1_1. For example, the signal GS1_i−1 (iis any one of 1 to N) goes into an H level. After that, when the signalGS2 and the signal SG3 are inverted, the signal GS1_i−1 goes into an Llevel and the signal GS1_i goes into an H level. After that, when thesignal GS2 and the signal SG3 are inverted again, the signal GS1_i goesinto an L level and a signal GS_i+1 goes into an H level. In thismanner, the signals GS1_1 to GS1_N sequentially go into an H level. Inother words, the wirings 201_1 to 201_N are sequentially selected.

The circuit 221 has a function of supplying a signal, voltage, or thelike to the shift register 220 to control the shift register 220 and canfunction as a control circuit or a controller or the like. In thisembodiment, the circuit 211 supplies the signal GS2, the signal GS3, thevoltage V1, the signal GS4, and the signal GS5 to the wiring 202, thewiring 203, the wiring 204, the wiring 205, and the wiring 206,respectively. However, this embodiment is not limited to this. The shiftregister 220 can supply a signal, current, voltage, or the like to avariety of circuits besides these wirings to control these circuits. Forexample, the circuit 221 can supply a signal, voltage, or the like to asignal line driver circuit, a scan line driver circuit, a pixel, and/orthe like to control these circuits.

The circuit 221 includes a circuit 222 and a circuit 223, for example.The circuit 222 has a function of generating power supply voltage suchas positive power supply voltage, negative power supply voltage, groundvoltage, or reference voltage and can function as a power supply circuitor a regulator. The circuit 223 has a function of generating a varietyof signals such as a clock signal, an inverted clock signal, a startsignal, a reset signal, and/or a video signal and can function as atiming generator. However, this embodiment is not limited to this. Thecircuit 221 can include a variety of circuits or a variety of elementsin addition to the circuit 222 and the circuit 223. For example, thecircuit 221 can include an oscillator, a level shifter circuit, aninverter circuit, a buffer circuit, a DA converter circuit, an ADconverter circuit, an operational amplifier, a shift register, a look-uptable, a coil, a transistor, a capacitor, a resistor, a frequencydivider, and/or the like.

Next, one example of the shift register 220 will be described withreference to FIG. 15. The shift register in FIG. 15 includes a pluralityof flip-flops of flip-flops 200_1 to 200_N (N is a natural number). Theflip-flops 200_1 to 200_N each correspond to the semiconductor devicedescribed in Embodiment 1 and Embodiment 2. FIG. 15 shows a structure inwhich the semiconductor device in FIG. 9B is used as a flip-flop as anexample.

Connection relations of the shift register are described. First, as anexample, a connection relation of the flip-flop 200_i is described. Inthe flip-flop 200_i, the wiring 121, the wiring 122, the wiring 123, thewiring 124, the wiring 126, and the wiring 127 are connected to thewiring 201_i, the wiring 204, the wiring 202, the wiring 203, the wiring201_i−1, and the wiring 201_i+1, respectively. However, the wirings tobe connected to the wiring 123 and the wiring 124 are switched in aflip-flop of an odd-numbered stage and a flip-flop of an even-numberedstage in many cases. For example, if the wiring 123 is connected to thewiring 202 and the wiring 124 is connected to the wiring 203 in theflip-flop of the odd-numbered stage, the wiring 123 is connected to thewiring 203 and the wiring 124 is connected to the wiring 202 in theflip-flop of the even-numbered stage. On the other hand, if the wiring123 is connected to the wiring 203 and the wiring 124 is connected tothe wiring 202 in the flip-flop of the odd-numbered stage, the wiring123 is connected to the wiring 202 and the wiring 124 is connected tothe wiring 203 in the flip-flop of the even-numbered stage.

Note that in the flip-flop 200_1, the wiring 125 is connected to thewiring 205.

Note that in the flip-flop 200_N, the wiring 126 is connected to thewiring 206.

Next, one example of operation of the shift register shown in FIG. 15 isdescribed with reference to a timing chart shown in FIG. 14B. Note thatdescription of operation which is the same as that of the semiconductordevice described in Embodiment 1 and Embodiment 2 is omitted.

Operation of the flip-flop 200_i is described. First, the signal GS1_i−1goes into an H level. Then, the flip-flop 200_i starts operation in theperiod T1 and the signal GS1_i goes into an L level. After that, thesignal GS2 and the signal GS3 are inverted. Then, the flip-flop 200_istarts operation in the period T2 and the signal GS1_i goes into an Hlevel. The signal GS1_i is input to the flip-flop 200_i−1 as a resetsignal and is input to the flip-flop 200_i+1 as a start signal.Accordingly, the flip-flop 200_i−1 starts operation in the period T3 andthe flip-flop 200_i+1 starts operation in the period T1. After that, thesignal GS2 and the signal GS3 are inverted again. Then, the flip-flop200_i+1 starts operation in the period T2 and the signal GS1_i+1 goesinto an H level. The signal GS1_i+1 is input to the flip-flop 200_i as areset signal. Accordingly, since the flip-flop 200_i starts operation inthe period T3, the signal GS1_i goes into an L level. After that, untilthe signal GS1_i−1 goes into an H level again, the flip-flop 200_irepeats the operation in the period T4 and the operation in the periodT5 every time the signal GS2 and the signal GS3 are inverted.

Note that, instead of an output signal from a flip-flop of the previousstage, the signal GS4 is input from an external circuit to the flip-flop200_1 through the wiring 205. Therefore, when the signal GS4 goes intoan H level, the flip-flop 200_1 starts operation in the period T1.

Note that, instead of an output signal from a flip-flop of the nextstage, the signal GS5 is input from an external circuit to the flip-flop200_N through the wiring 206. Therefore, when the signal GS5 goes intoan H level, the flip-flop 200_N starts operation in the period T3.

By using the semiconductor device described in Embodiment 1 andEmbodiment 2 as the shift register in this embodiment, a similaradvantage as the semiconductor device described in Embodiment 1 andEmbodiment 2 can be obtained.

Note that the wiring 206 can be eliminated. In that case, for example, astructure in which the transistor 134 and the transistor 135 which areshown in FIG. 10C are eliminated can be employed for the flip-flop200_N.

Note that in the case where a signal is used instead of the voltage V1in the flip-flops 200_1 to 200_N, the wiring 204 can be eliminated.

Note that the signal GS4 can be input to the wiring 206 as in the caseof the wiring 205. In that case, by connecting the wiring 206 to thewiring 205, the wiring 205 and the wiring 206 can be shared.Alternatively, the signal GS2 can be input to the wiring 206 as in thecase of the wiring 202. In that case, by connecting the wiring 206 tothe wiring 202, the wiring 206 and the wiring 202 can be shared. Furtheralternatively, the signal GS3 can be input to the wiring 206 as in thewiring 203. In that case, by connecting the wiring 206 to the wiring203, the wiring 206 and the wiring 203 can be shared. Alternatively, thevoltage V1 can be input to the wiring 206 as in the case of the wiring204. In that case, by connecting the wiring 206 to the wiring 204, thewiring 206 and the wiring 204 can be shared.

Note that in the case where a structure which requires the signal GS6like that in FIG. 13B is used for the flip-flops 200_1 to 200_N, awiring 207 can be added as shown in FIG. 16. The signal GS6 is input tothe wiring 207. The signal GS6 corresponds to the signal S6 described inEmbodiment 2 and can function as a reset signal in all stages. Inaddition, the wiring 207 corresponds to the wiring 128 in FIG. 13B andcan function as a signal line.

However, this embodiment is not limited to this. By making the wiring207 and a different wiring shared, the number of wirings or the numberof signals or power supply voltages can be reduced. For example, thesignal GS4 can be input to the wiring 207 as in the case of the wiring205. Accordingly, by connecting the wiring 207 to the wiring 205, thewiring 207 and the wiring 205 can be shared. Alternatively, the signalGS5 can be input to the wiring 207 as in the case of the wiring 206.Accordingly, by connecting the wiring 207 to the wiring 206, the wiring207 and the wiring 206 can be shared. Alternatively, a signal S1_N,which is an output signal from the flip-flop 200_N, can be input to thewiring 207. Accordingly, by connecting the wiring 207 to the wiring201_N, the wiring 207 and the wiring 201_N can be shared.

Note that in the case where a structure which requires the voltage V2like that in FIG. 9C is used for the flip-flops 200_1 to 200_N, a wiringcan be additionally provided. The voltage V2 is supplied to the wiringthat is additionally provided. In addition, the wiring corresponds tothe wiring 127 in FIG. 9C and can function as a power supply line.

Note that as described in Embodiment 1 and Embodiment 2, in the casewhere a signal whose potential in an L level is lower than V1, a signalwhose potential in an H level is lower than V2, or a signal whoseamplitude voltage is lower than (V2−V1), or the like is input to theflip-flop in order to suppress deterioration of characteristics of thetransistor, a wiring can be additionally provided. A signal is input tothe wiring. The wiring can function as a signal line.

Note that as shown in FIG. 17A, the shift register can include a circuit212, a circuit 213, a circuit 214, a circuit 215, and/or a circuit 216.The circuits 212 to 216 each have a function of increasing (ordecreasing) the amplitude voltage or input voltage of an input signaland outputting the input signal and can function as a level shiftercircuit. Alternatively, the circuits 212 to 216 have a function ofinverting an input signal and outputting the inverted input signal andcan function as an inverter circuit or a buffer circuit. The wiring 202is connected to the flip-flops through the circuit 212. The wiring 203is connected to the flip-flops through the circuit 213. The wiring 204is connected to the flip-flops through the circuit 214. The wiring 205is connected to the flip-flops through the circuit 215. The wiring 206is connected to the flip-flops through the circuit 216. In this manner,since a signal with low amplitude can be input to the shift register,the driving voltage of an external circuit can be reduced. Therefore,reduction in cost, power consumption, or the like of the externalcircuit can be achieved.

Note that as shown in FIG. 17A, the shift register can include any one,two, or more of the circuits 212 to 216.

Note that as shown in FIG. 17B, the shift register can include circuits211_1 to 211_N. The circuits 211_1 to 211_N each have a function ofincreasing the current capability of an input signal, increasing theamplitude voltage of the input signal, or inverting the input signal,and can function as a buffer circuit, a level shifter circuit, or aninverter circuit. The circuits 211_1 to 211_N are connected between therespective flip-flops 200_1 to 200_N and the respective wirings 201_1 to201_N. For example, the circuit 211_i is connected between the flip-flop200_i and the wiring 201_i. Then, the signal GS1_i which is an outputsignal from the flip-flop 200_i is output from the wiring 201_i throughthe circuit 211_i. In this manner, since the driving voltage of eachflip-flop can be made low, reduction in power consumption, suppressionof deterioration in characteristics of a transistor, or the like can beachieved. Alternatively, since the channel width of a transistor (inspecific, the transistor 101) included in each flip-flop can be madesmall, reduction in a layout area can be achieved.

Note that in an example shown in FIG. 17B, the signal GS1_i is input asa reset signal to the flip-flop 200_i−1 through the circuit 211_i.Therefore, in the flip-flop 200_i−1, since a period of time when thetransistor 101 is on is long during the period T3, falling time of thesignal GS_i−1 which is an output signal from the flip-flop 200_i−1 canbe shortened. On the other hand, the signal GS1_i is input as a startsignal to the 200_i+1 without passing through the circuit 211_i.Therefore, in the flip-flop 200_i+1, since the potential of the node Acan be quickly raised during the period T1, increase in drivingfrequency can be achieved. However, this embodiment is not limited tothis. The signal GS1_i can be input as a reset signal to the flip-flop200_i−1 without passing through the circuit 211_i. Alternatively, thesignal GS1_i can be input as a start signal to the flip-flop 200_i+1through the circuit 211_i.

Note that in the shift register shown in FIG. 14A, the cycles of signalsGS1_1 to GS1_N are different from each other by half a cycle of thesignal S2 or by half a cycle of the signal S3. However, this embodimentis not limited to this. The cycles of the signals GS1_1 to GS1_N can bedifferent from each other by ½×M (M is a natural number) of a cycle ofthe signal S2 or by ½×M of a cycle of the signal S3. That is, a periodin which a signal among the signals GS1_1 to GS1_N in one row goes intoan H level and a period in which a signal among the signals GS1_1 toGS1_N in a different row goes into an H level can overlap with eachother. In order to realize this, a clock signal with a phase of 2×M canbe input to the shift register.

A specific example is described with reference to a shift register inFIG. 24. FIG. 24 shows only the flip-flops 200_i+1 to 200_i+2M+1. Thewirings 123 of the flip-flops 200_i+1 to 200_i+M are connected towirings 203_1 to 203_M, respectively. The wirings 124 of the flip-flops200_i+1 to 200_i+M are connected to wirings 204_1 to 204_M,respectively. The wirings 123 of the flip-flops 200_i+M+1 to 200_i+2Mare connected to the wirings 204_1 to 204_M, respectively. The wirings124 of the flip-flops 200_i+M+1 to 200_i+2M are connected to the wirings203_1 to 203_M, respectively. In addition, the wiring 125 of theflip-flop 200_i+1 is connected to the wiring 121 of the flip-flop 200_i.The wiring 126 of the flip-flop 200_i+1 is connected to the wiring 121of the flip-flop 200_i+M+1. Note that the wirings 203_1 to 203_Mcorrespond to the wiring 203. The wirings 204_1 to 204_M correspond tothe wiring 204. As shown in FIG. 25A, signals GS2_1 to GS2_M are inputto the wirings 203_1 to 203_M, respectively. Signals GS3_1 to GS3_M areinput to the wirings 204_1 to 204_M, respectively. The signals GS2_1 toGS2_M are M clock signals whose phases are different from each other by½M of a cycle and correspond to the signal GS2. The signals GS3_1 toGS3_M are inverted signals of the signals GS2_1 to GS2_M and correspondto the signal GS3. In this manner, the cycles of the signals S1_1 toS1_N can be different from that of the signal S2 by ½×M (M is a naturalnumber) of a cycle or different from that of the signal S3 by ½×M of acycle.

Note that in FIG. 24, the wiring 125 of the flip-flop 200_i+1 can beconnected the wiring 121 of any one of the flip flops 200_i−M+1 to200_i−1. In this manner, since a timing when the transistor 131 in theflip-flop 200_i+1 is turned on can come up earlier, a timing when thepotential of the node A is raised can come up earlier. Therefore,driving frequency can be increased. Alternatively, since the channelwidth of the transistor 131 or the transistor 132 can be reduced,reduction in a layout area can be achieved.

Note that in FIG. 24, the wiring 126 of the flip-flop 200_i+1 can beconnected to the wiring 121 of any one of the flip-flops 200_i+M+2 to200_i+2M. In this manner, a timing when the transistor 101 in theflip-flop 200_i+1 is turned off can come up later, falling time of thesignal S+1 can be shortened.

Note that in FIG. 24, the wiring 126 of the flip-flop 200_i+1 can beconnected to the wiring 121 of any one of the flip-flops 200_i+2 to200_i+M. In this manner, the pulse width of the signals S1_1 to S1_N canbe made smaller than that of half a cycle of the clock signal.Therefore, driving frequency can be increased while reduction in powerconsumption is achieved.

Note that in FIG. 24, it is preferable that 4. It is more preferablethat 2. This is because, in the case where a shift register in FIGS. 23Ato 23E is used for a scan line driver circuit of a display device,plural kinds of video signals are written to a pixel if M is too large anumber. Therefore, a period in which an irregular video signal is inputto the pixel gets long and display quality is impaired in some cases.FIG. 25B shows an example of a timing chart in the case where M=2.

Embodiment 4

In this embodiment, an example of a semiconductor device and a shiftregister including the semiconductor device will be described. Note thatthe content described in Embodiments 1 to 3 can be applied to that ofthe semiconductor device and the shift register in this embodiment.

First, the semiconductor device of this embodiment will be describedwith reference to FIG. 19A. Note that portions common to those of FIG.1A are denoted by common reference numerals, and thus descriptionthereof is omitted.

The semiconductor device in FIG. 19A includes the circuit 100, thetransistor 101, the transistor 102, the transistor 103, the transistor104, the capacitor 105, the capacitor 106, and a transistor 301. Thetransistor 301 corresponds to the transistor 101 and has a similarfunction to the transistor 101. In addition, the transistor 301 is ann-channel transistor. Note that the transistor 301 can be a p-channeltransistor.

A first terminal of the transistor 301 is connected to a wiring 123D. Asecond terminal of the transistor 301 is connected to a wiring 311. Agate of the transistor 301 is connected to the node A.

The wiring 123D corresponds to the wirings 123A to 123C. The signal S2is input to the wiring 123D. Accordingly, as in FIG. 3D, the wiring 123Dand the wirings 123A to 123C can be shared. In that case, the firstterminal of the transistor 301 is connected to the wiring 123. A signalS7 is output from the wiring 311. The signal S7 corresponds to thesignal S1.

Next, operation of the semiconductor device in FIG. 19A is describedwith reference to a timing chart in FIG. 19B. Note that description ofoperation in common with the semiconductor device in FIG. 1A is omitted.

First, the potential of the node A starts to be raised in the period T1.Then, like the transistor 101, the transistor 301 is turned on when thepotential of the node A becomes equal to the sum of the potential (V1)of the wiring 123D and the threshold voltage (Vth301) of the transistor301, (V1+Vth301). Then, the wiring 123D and the wiring 311 are broughtinto electrical conduction. Therefore, since the signal S2 in an L levelis supplied from the wiring 123D to the wiring 311, the potential of thewiring 311 is decreased to V1.

Next, since the potential of the node A gets (V1+Vth101+α) in the periodT2, the transistor 301 is kept on. Accordingly, the wiring 123D and thewiring 311 are kept in electrical conduction. Therefore, since thesignal S2 in an H level is supplied from the wiring 123D to the wiring311, the potential of the wiring 311 is raised to V2.

Next, the potential of the node A starts to decrease to V1 in the periodT3. Like the transistor 101, the transistor 301 is on until thepotential of the node A becomes equal to the sum of the potential (V1)of the wiring 123D and the threshold voltage (Vth301) of the transistor301, (V1+Vth301). Therefore, since the signal S1 in an L level issupplied from the wiring 123D to the wiring 311, the potential of thewiring 311 is decreased to V1. After that, when the potential of thenode A is decreased to (V1+Vth301), the transistor 301 is turned off.

During the period T4 and the period T5, since the potential of the nodeA is maintained as V1, the transistor 301 is kept off. Therefore, thewiring 123D and the wiring 311 are kept out of electrical conduction.

In the semiconductor device in FIG. 19A, the wiring 121 and the wiring311 can output signals with the same timings. Therefore, one of thesignal S1 output from the wiring 121 and the signal S7 output from thewiring 311 can be used for driving a load such as a gate line or a pixeland the other thereof can be used as a signal for driving a differentcircuit, such as a signal for transferring. In this manner, thedifferent circuit can be driven without being adversely influenced bydistortion, delay, or the like of a signal caused by driving the load orthe like.

Note that a capacitor can be connected between the gate and the secondterminal of the transistor 301. The capacitor corresponds to thecapacitor 105.

Note that as shown in FIG. 20A, the transistor 301 can be added to thesemiconductor device in FIG. 6A.

Note that as shown in FIG. 20B, a transistor 302, a transistor 303,and/or a transistor 304 can be added. The transistor 302, the transistor303, and the transistor 304 correspond and have similar functions to thetransistor 134, the transistor 102, and the transistor 133,respectively. A first terminal of the transistor 302 is connected to awiring 122H. A second terminal of the transistor 302 is connected to awiring 331. A gate of the transistor 302 is connected to the wiring 126.A first terminal of the transistor 303 is connected to the wiring 331. Asecond terminal of the transistor 303 is connected to the node A. A gateof the transistor 303 is connected to a wiring 123E. A first terminal ofthe transistor 304 is connected to a wiring 122I. A second terminal ofthe transistor 304 is connected to the wiring 331. A gate of thetransistor 304 is connected to a wiring 124C. However, this embodimentis not limited to this. Only one or two of the transistor 302, thetransistor 303, and the transistor 304 can be added.

Note that in FIG. 20B, since the same signal (e.g., the signal S2) asthe wirings 123A to 123C is input to the wiring 123D and the wiring123E, the wiring 123D, the wiring 123E, and the wirings 123A to 123C canbe shared. In that case, the first terminal of the transistor 301 andthe gate of the transistor 303 are connected to the wiring 123.

Note that in FIG. 20B, since the same voltage (e.g., the voltage V1) asthe wirings 122A to 122E is input to the wiring 122H and the wiring122I, the wiring 122H, the wiring 122I, and the wirings 122A to 122E canbe shared. In that case, the first terminal of the transistor 302 andthe gate of the transistor 304 are connected to the wiring 122.

Note that in FIG. 20B, like the transistor 135, the transistor 302 canbe replaced with a diode or a diode-connected transistor. Alternatively,like the transistor 133, the transistor 304 can be replaced with a diodeor a diode-connected transistor.

Next, one example of a shift register including the above-describedsemiconductor device is described with reference to FIG. 21. Note thatdescription of the content described in Embodiment 3 is omitted.Alternatively, the same portions as those in FIG. 14 are denoted by thesame reference numerals and description thereof is omitted.

The shift register includes a plurality of flip-flops of flip-flops320_1 to 320_N. The flip-flops 320_1 to 320_N correspond to theflip-flops 200_1 to 200_N in FIG. 14. Alternatively, the flip-flops320_1 to 320_N correspond to the semiconductor device in FIG. 19A, FIG.20A, or FIG. 20B. FIG. 21 shows one example of the case where thesemiconductor device in FIG. 20A is used.

In the flip-flop 320_i, the wiring 311 is connected to the wiring 321_i.Then, the wiring 126 is connected to the wiring 321_i−1.

Signals GS7_1 to GS7_N are output from the wirings 321_1 to 321_N,respectively. The signals GS7_1 to GS7_N correspond to the signal S7 andeach can function as a transfer signal, an output signal, a selectionsignal, a scan signal, or a gate signal.

Next, operation of the shift register shown in FIG. 21 is described withreference to the timing chart in FIG. 14B.

Operation of the flip-flop 320_i is described. First, the signal GS7_i−1goes into an H level. Then, the flip-flop 320_i starts operation in theperiod T2 and the signal GS1_i and the signal GS7_i go into an L level.After that, the signal GS2 and the signal GS3 are inverted. Then, theflip-flop 320_i starts operation in the period T2 and the signal GS1_iand the signal GS7_i go into an H level. The signal GS1_i is input tothe flip-flop 320_i−1 as a reset signal and the signal GS7_i is input tothe flip-flop 320_i+1 as a start signal. Accordingly, the flip-flop320_i−1 starts operation in the period T3 and the flip-flop 320_i+1starts operation in the period T1. After that, the signal GS2 and thesignal GS3 are inverted again. Then, the flip-flop 320_i+1 startsoperation in the period T2 and the signal GS1_i+1 goes into an H level.The signal GS1_i+1 is input to the flip-flop 320_i as a reset signal.Accordingly, since the flip-flop 320_i starts operation in the periodT3, the signal GS1_i and the signal GS7_i go into an L level. Afterthat, until the signal GS7_i−1 goes into the H level again, theflip-flop 320_i repeats the operation in the period T4 and the operationin the period T5 every time the signal GS2 and the signal GS3 areinverted.

In the shift register in this embodiment, since the signals GS7_1 toGS7_N are used as start signals, delay time of the signals S1_1 to S1_Ncan be shortened. This is because, since the signals GS7_1 to GS7_N arenot input to the gate line, the pixel, or the like, delay or distortionof the signals GS7_1 to GS7_N is slight as compared to the signals S1_1to S1_N.

Alternatively, in the shift register of this embodiment, since thesignals GS1_1 to GS1_N are used as reset signals, a period of time whenthe transistor 101 is on in operation of each flip-flop during theperiod T3 can be made longer. Therefore, falling time of the signalsS1_1 to S1_N and falling time of the signals GS7_1 to GS7_N can beshortened.

Note that the signals GS1_1 to GS1_N can be input to the flip-flop inthe next stage as start signals. For example, the signal GS1_i can beinput to the flip-flop 320_i+1 as a start signal.

Note that the signals GS7_1 to GS7_N can be input to the flip-flop inthe previous stage as reset signals. For example, the signal GS7_i canbe input to the flip-flop 320_i−1 as a reset signal.

Embodiment 5

In this embodiment, an example of a display device is described. First,an example of a system block of a liquid crystal display device isdescribed with reference to FIG. 22A. The liquid crystal display deviceincludes a circuit 5361, a circuit 5362, a circuit 5363_1, a circuit5363_2, a pixel portion 5364, a circuit 5365, and a lighting device5366. A plurality of wirings 5371 which is extended from the circuit5362 and a plurality of wirings 5372 which is extended from the circuit5363_1 and the circuit 5363_2 are provided in the pixel portion 5364. Inaddition, pixels 5367 which include display elements such as liquidcrystal elements are provided in matrix in respective regions where theplurality of wirings 5371 and the plurality of wirings 5372 intersectwith each other.

The circuit 5361 has a function of outputting a signal, voltage, or thelike to the circuit 5362, the circuit 5363_1, the circuit 5363_2, andthe circuit 5365 in response to a video signal 5360 and can function asa controller, a control circuit, a timing generator, a regulator, or thelike.

For example, the circuit 5361 outputs a signal such as a signal linedriver circuit start signal (SSP), a signal line driver circuit clocksignal (SCK), a signal line driver circuit inverted clock signal (SCKB),a video signal data (DATA), or a latch signal (LAT) to the circuit 5362.The circuit 5362 has a function of outputting video signals to theplurality of wirings 5371 in response to such a signal and functions asa signal line driver circuit.

Note that in the case where the video signals are input to the pluralityof wirings 5371, the plurality of wirings 5371 can function as signallines, video signal lines, source lines, or the like.

For example, the circuit 5361 outputs a signal such as a scan linedriver circuit start signal (GSP), a scan line driver circuit clocksignal (GCK), or a scan line driver circuit inverted clock signal (GCKB)to the circuit 5363_1 and the circuit 5363_2. The circuit 5363_1 and thecircuit 5363_2 each have a function of outputting scan signals to theplurality of wirings 5372 in response to such a signal and function as ascan line driver circuit.

Note that in the case where scan signals are input to the plurality ofwirings 5372, the plurality of wirings 5372 can function as signallines, scan lines, gate lines, or the like.

Note that since the same signal is input to the circuit 5363_1 and thecircuit 5363_2 from the circuit 5361, scan signals output from thecircuit 5363_1 to the plurality of wirings 5367 and scan signals outputfrom the circuit 5363_2 to the plurality of wirings 5367 haveapproximately the same timings in many cases. Therefore, load caused bydriving of the circuit 5363_1 and the circuit 5363_2 can be reduced.

Accordingly, the display device can be made larger. Alternatively, thedisplay device can have higher definition. Alternatively, since thechannel width of transistors included in the circuit 5363_1 and thecircuit 5363_2 can be reduced, a display device with a narrower framecan be obtained.

For example, the circuit 5361 outputs a backlight control signal (BLC)to the circuit 5365. The circuit 5365 has a function of controlling theluminance (or the average luminance) of the lighting device 5366 bycontrolling the amount of electric power supplied to the lighting device5366, time to supply the electric power to the lighting device 5366, orthe like in accordance with the backlight control signal (BLC) andfunctions as a power supply circuit.

Note that one of the circuit 5363_1 and the circuit 5363_2 can beeliminated.

Note that a wiring such as a capacitor line, a power supply line, or ascan line can be newly provided in the pixel portion 5364. Then, thecircuit 5361 can output a signal, voltage, or the like to such a wiring.In addition, a circuit similar to the circuit 5363_1 or the circuit5363_2 can be additionally provided. The additionally provided circuitcan output a signal such as a scan signal to the additionally providedwiring.

Note that the pixel 5367 can include a light-emitting element such as anEL element as a display element. In that case, as shown in FIG. 22B,since the display element emits light, the circuit 5365 and the lightingdevice 5366 can be eliminated. In addition, in order to supply electricpower to the display element, a plurality of wirings 5373 which canfunction as power supply lines can be provided in the pixel portion5364. The circuit 5361 can supply power supply voltage called voltage(ANO) to the wirings 5373. The wirings 5373 can be separately connectedthe pixels in accordance with color elements or connected to all thepixels.

Note that FIG. 22B shows an example in which the circuit 5361 suppliesdifferent signals to the circuit 5363_1 and the circuit 5363_2. Thecircuit 5361 outputs a signal such as a scan line driver circuit startsignal (GSP1), a scan line driver circuit clock signal (GCK1), and ascan line driver circuit inverted clock signal (GCKB1) to the circuit5363_1. In addition, the circuit 5361 outputs a signal such as a scanline driver circuit start signal (GSP2), a scan line driver circuitclock signal (GCK2), and a scan line driver circuit inverted clocksignal (GCKB2) to the circuit 5363_2. In that case, the circuit 5363_1can scan only wirings in odd-numbered rows of the plurality of wirings5372 and the circuit 5363_2 can scan only wirings in even-numbered rowsof the plurality of wirings 5372. Accordingly, the driving frequency ofthe circuit 5363_1 and the circuit 5363_2 can be lowered, wherebyreduction in power consumption can be achieved. Alternatively, an areain which a flip-flop of one stage can be laid out can be made larger.Therefore, a display device can have higher definition. Alternatively, adisplay device can be made larger.

Note that as in FIG. 22B, the circuit 5361 can supply different signalsto the circuit 5363_1 and the circuit 5363_2 in FIG. 22A.

Next, one example of a structure of the display device are describedwith reference to FIGS. 23A to 23E.

In FIG. 23A, circuits which have a function of outputting signals to thepixel portion 5364 (e.g., the circuit 5362, the circuit 5363_1, and thecircuit 5363_2) are formed over a substrate 5380 over which the pixelportion 5364 is also formed. In addition, the circuit 5361 is formedover a different substrate from the pixel portion 5364. In this manner,since the number of external components is reduced, reduction in costcan be achieved. Alternatively, since the number of signals or voltagesinput to the substrate 5380 is reduced, the number of connectionsbetween the substrate 5380 and the external component can be reduced.Therefore, improvement in reliability or increase in yield can beachieved.

Note that in the case where the circuit is formed over a differentsubstrate from the pixel portion 5364, the substrate can be mounted onan FPC (flexible printed circuit) by a TAB (tape automated bonding)method. Alternatively, the substrate can be mounted on the samesubstrate 5380 as the pixel portion 5364 by a COG (chip on glass)method.

Note that in the case where the circuit is formed over a differentsubstrate from the pixel portion 5364, a transistor formed using asingle crystal semiconductor can be formed on the substrate. Therefore,the circuit formed over the substrate can have advantages such asimprovement in driving frequency, improvement in driving voltage, orsuppression of variations in output signals.

Note that a signal, voltage, current, or the like is input from anexternal circuit through an input terminal 5381 in many cases.

In FIG. 23B, circuits with low driving frequency (e.g., the circuit5363_1 and the circuit 5363_2) are formed over the substrate 5380 as thepixel portion 5364. In addition, the circuit 5361 and the circuit 5362are formed over a different substrate from the pixel portion 5364. Inthis manner, since the circuit formed over the substrate 5380 can beformed using a transistor with low mobility, a non-single-crystalsemiconductor, a microcrystalline semiconductor, an organicsemiconductor, an oxide semiconductor, or the like can be used for asemiconductor layer of the transistor. Accordingly, increase in the sizeof the display device, reduction in the number of steps, reduction incost, improvement in yield, or the like can be achieved.

Note that as shown in FIG. 23C, part of the circuit 5362 (a circuit 5362a) can be formed over the substrate 5380 over which the pixel portion5364 is formed and the other part of the circuit 5362 (a circuit 5362 b)can be formed over a different substrate from the pixel portion 5364.The circuit 5362 a includes a circuit which can be formed using atransistor with low mobility in many cases (e.g., a shift register, aselector, or a switch). In addition, the circuit 5362 b includes acircuit which is preferably formed using a transistor with high mobilityand few variations in characteristics in many cases (e.g., a shiftregister, a latch circuit, a buffer circuit, a DA converter circuit, oran AD converter circuit). In this manner, as in FIG. 23B, anon-single-crystal semiconductor, a microcrystalline semiconductor, anorganic semiconductor, an oxide semiconductor, or the like can be usedfor a semiconductor layer of the transistor. Further, reduction inexternal components can be achieved.

In FIG. 23D, circuits which have a function of outputting signals to thepixel portion 5364 (e.g., the circuit 5362, the circuit 5363_1, and thecircuit 5363_2) and a circuit which has a function of controlling thesecircuits (e.g., the circuit 5361) are formed over a different substratefrom the pixel portion 5364. In this manner, since the pixel portion andperipheral circuits thereof can be formed over different substrates,improvement in yield can be achieved.

In FIG. 23E, part of the circuit 5361 (a circuit 5361 a) is formed overthe substrate 5380 over which the pixel portion 5364 and the other partof the circuit 5361 (a circuit 5361 b) is formed over a differentsubstrate from the pixel portion 5364. The circuit 5361 a includes acircuit which can be formed using a transistor with low mobility in manycases (e.g., a switch, a selector, or a level shift circuit). Inaddition, the circuit 5361 b includes a circuit which is preferablyformed using a transistor with high mobility and few variations in manycases (e.g., a shift register, a timing generator, an oscillator, aregulator, or an analog buffer).

Note that as the circuit 5363_1 and the circuit 5363_2, thesemiconductor device or shift register in Embodiments 1 to 4 can beused. In that case, if the circuit 5363_1 and the circuit 5363_2 areformed over the same substrate as the pixel portion, the polarity of alltransistors formed over the substrate can be n-type or p-type.Accordingly, reduction in the number of steps, improvement in yield, orreduction in cost can be achieved. In specific, by setting thepolarities of all the transistors n-type, a non-single-crystalsemiconductor, a microcrystalline semiconductor, an organicsemiconductor, an oxide semiconductor, or the like can be used for asemiconductor layer of the transistor. Therefore, increase in the sizeof the display device, reduction in cost, improvement in yield, or thelike can be achieved.

Note that deterioration of characteristics, such as increase inthreshold voltage or decrease in mobility, is caused in many cases inthe transistor whose semiconductor layer is formed using anon-single-crystal semiconductor, a microcrystalline semiconductor, anorganic semiconductor, an oxide semiconductor, or the like. However,since deterioration of the characteristics of the transistor can besuppressed in the semiconductor device or shift register in Embodiments1 to 4, the life of the display device can be made longer.

Note that the semiconductor device or shift register in Embodiments 1 to4 can be used as part of the circuit 5362. For example, the circuit 5362a shown in FIG. 23C can include the semiconductor device or shiftregister in Embodiments 1 to 4.

Embodiment 6

In this embodiment, a layout view (hereinafter also referred to as a topview) of a shift register will be described. In this embodiment, forexample, a layout view of the shift register shown in FIG. 15 will bedescribed. Note that a content described in this embodiment can beapplied to the semiconductor device, the shift register, or the displaydevice in Embodiments 1 to 5 in addition to the shift register in FIG.15. Note that the layout view in this embodiment is one example and thisembodiment is not limited to this.

The layout view in this embodiment is described with reference to FIG.30 and FIG. 31. FIG. 30 illustrates one example of a layout view of partof the shift register. FIG. 31 illustrates a layout view of theflip-flop 200 i, for example.

A transistor, a capacitor, a wiring, and the like shown in FIG. 30 andFIG. 31 each include a conductive layer 401, a semiconductor layer 402,a conductive layer 403, a conductive layer 404, and a contact hole 405.However, this embodiment is not limited to this. A different conductivelayer, insulating film, or a different contact hole can be newly formed.For example, a contact hole which connects the conductive layer 401 tothe conductive layer 403 can be additionally provided.

The conductive layer 401 can include a portion which functions as a gateelectrode or a wiring. The semiconductor layer 402 can include a portionwhich functions as a semiconductor layer of the transistor. Theconductive layer 403 can include a portion which functions as a wiringor a source or drain. The conductive layer 404 can include a portionwhich functions as a transparent electrode, a pixel electrode, or awiring. The contact hole 405 can be used in order to connect theconductive layer 401 and the conductive layer 404 or in order to connectthe conductive layer 403 and the conductive layer 404.

In the example in FIG. 30, the wiring 202 includes an opening portion411 and the wiring 203 includes an opening portion 412. In this manner,since the wiring 202 and the wiring 203 include the opening portions,parasitic capacitance can be reduced. Alternatively, breakdown of thetransistor due to electrostatic discharge can be suppressed. However,this embodiment is not limited to this. Like the wiring 204, the openingportion 411 or the opening portion 412 can be eliminated. Alternatively,like the wiring 202 or the wiring 203, an opening portion can beprovided for the wiring 204.

In the example in FIG. 30, by providing an opening portion in part of anintersection portion of the wiring 202 or the wiring 203 and a differentwiring, the cross-over capacitance of the wiring can be reduced.Accordingly, reduction in noise or reduction in delay or distortion of asignal can be achieved.

In the example in FIG. 30, the conductive layer 404 is formed over partof the conductive layer 403 included in the wiring 204. Then, theconductive layer 404 is connected to the conductive layer 403 throughthe contact hole 405. In this manner, since wiring resistance can bemade low, suppression of voltage drop or reduction in delay ordistortion of a signal can be achieved. However, this embodiment is notlimited to this. The conductive layer 404 and the contact hole 405 canbe eliminated. Alternatively, like the wiring 204, the conductive layer404 can be formed over part of the conductive layer 403 in the wiring202 or the wiring 203 so that the conductive layer 404 can be connectedto the conductive layer 403.

Here, in the example in FIG. 30, the width of the wiring 202, the widthof the wiring 203, and the width of the wiring 204 are denoted as wiringwidth 421, wiring width 422, and width 423, respectively. Then, thewidth of the opening portion 411, the length of the opening portion 411,the width of the opening portion 412, and the length of the openingportion 412 are denoted as width 424, length 425, width 426, and length427, respectively.

Note that signals input to the wiring 202 and the wiring 203 areinverted signals of each other in many cases. Therefore, the wiringresistance or the parasitic capacitance of the wiring 202 is preferablyset so as to be approximately equal to that of the wiring 203.Accordingly, the wiring 202 preferably includes a portion whose width isapproximately equal to the wiring width 422. Alternatively, the openingportion 411 preferably includes a portion whose width is approximatelyequal to the width 426 of the opening portion 412 or a portion whoselength is approximately equal to the length 427 of the opening portion412. However, this embodiment is not limited to this. The wiring width421, the wiring width 422, the width 424 of the opening portion 411, thelength 425 of the opening portion 411, or the length 427 of the openingportion 412 can have a variety of values. For example, when thecross-over capacitance of the wiring 202 and a different wiring ishigher than that of the wiring 203 and a different wiring, delay ordistortion of signals input to the wiring 202 and the wiring 203 can bemade approximately the same by decreasing the wiring resistance of thewiring 202. Therefore, the wiring 202 can include a portion which iswider than the wiring width 422. Alternatively, the opening portion 411can include a portion which is narrower than the width 426 of theopening portion 412. Alternatively, the opening portion 411 can includea portion which is shorter than the length 427 of the opening portion412. On the other hand, when the cross-over capacitance of the wiring202 and a different wiring is lower than that of the wiring 203 and adifferent wiring, the wiring 202 can include a portion whish is narrowerthan the wiring width 422. Alternatively, the opening portion 411 caninclude a portion which is wider than the width 426 of the openingportion 412. Alternatively, the opening portion 411 can include aportion which is longer than the length 427 of the opening portion 412.

In the case where the wiring 204 does not include the opening portion,the wiring 204 preferably includes a portion which is narrower than thewiring width 421 or the wiring width 422. This is because the wiring 204does not include an opening portion, and the wiring resistance of thewiring 204 is low. However, this embodiment is not limited to this. Thewiring 204 can include a portion which is wider than the wiring width421 or the wiring width 422.

In the example in FIG. 31, one electrode of each of the capacitor 105and the capacitor 106 is formed using the conductive layer 401 and theother electrode of each of the capacitor 105 and the capacitor 106 isformed using a conductive layer 403. Accordingly, since a capacitancevalue per unit area can be large, reduction in a layout area can beachieved. However, this embodiment is not limited to this. Thesemiconductor layer 402 can be provided between the conductive layer 401and the conductive layer 403. In this manner, short circuit between theconductive layer 401 and the conductive layer 403 can be suppressed.Alternatively, the capacitor 105 or the capacitor 106 may be a MOScapacitor.

In the example in FIG. 31, in the transistor 101, the transistor 103,the transistor 104, the transistor 131, the transistor 132, thetransistor 133, the transistor 134, and the transistor 135, an areawhere the conductive layers 401 and the conductive layers 403 of thesecond terminals overlap with each other is preferably smaller than anarea where the conductive layers 401 and the conductive layers 403 ofthe first terminals overlap with each other. In this manner, reductionin noise of the gate of the transistor 101 or the wiring 201_i can beachieved. Alternatively, since concentration of an electric field on thesecond terminal can be suppressed, deterioration of the transistor orbreakdown of the transistor can be suppressed.

Note that the semiconductor layer 402 can be provided for a portion inwhich the conductive layer 401 and the conductive layer 403 overlap witheach other. Accordingly, the parasitic capacitance between theconductive layer 401 and the conductive layer 403 can be reduced,whereby reduction in noise can be achieved. Because of a similar reason,the semiconductor layer 402 or the conductive layer 403 can be providedfor a portion in which the conductive layer 401 and the conductive layer404 overlap with each other.

Note that the conductive layer 404 can be formed over part of theconductive layer 401 and can be connected to the conductive layer 401through the contact hole 405. Accordingly, wiring resistance can bereduced. Alternatively, the conductive layer 403 and the conductivelayer 404 can be formed over part of the conductive layer 401, so thatthe conductive layer 401 is connected to the conductive layer 404through the contact hole 405 and the conductive layer 403 can beconnected to the conductive layer 404 through the different contact hole405. In this manner, the wiring resistance can be further reduced.

Note that the conductive layer 404 can be formed over part of theconductive layer 403, so that the conductive layer 403 can be connectedto the conductive layer 404 through the contact hole 405. In thismanner, wiring resistance can be reduced.

Note that the conductive layer 401 or the conductive layer 403 can beformed under part of the conductive layer 404, so that the conductivelayer 404 can be connected to the conductive layer 401 or the conductivelayer 403 through the contact hole 405. In this manner, wiringresistance can be reduced.

Note that in the case where the capacitor 105 is eliminated, asdescribed in Embodiment 1, the parasitic capacitance between the gateand the second terminal of the transistor 101 can be higher than theparasitic capacitance between the gate and the first terminal of thetransistor 101. One example of a layout view of the transistor 101 inthat case is shown in FIG. 18. In the example in FIG. 18, the width ofthe conductive layer 403 which can function as a first electrode of thetransistor 101 is referred to as width 431 and the width of theconductive layer 403 which can function as a second electrode of thetransistor 101 is referred to as width 432. In addition, the width 431can be larger than the width 432. In this manner, as described inEmbodiment 1, the parasitic capacitance between the gate and the secondterminal of the transistor 101 can be higher than the parasiticcapacitance between the gate and the first terminal of the transistor101. However, this embodiment is not limited to this.

Embodiment 7

In this embodiment, one example of a signal line driver circuit will bedescribed. Note that the signal line driver circuit can be referred toas a semiconductor device or a signal generation circuit.

One example of the signal line driver circuit is described withreference to FIG. 26A. The signal line driver circuit includes aplurality of circuits of circuits 502_1 to 502_N (N is a naturalnumber), a circuit 500, and a circuit 501. In addition, the circuits502_1 to 502_N each include a plurality of transistors of transistors503_1 to 503_k (k is a natural number). The transistors 503_1 to 503_kare n-channel transistors. However, this embodiment is not limited tothis. The transistors 503_1 to 503_k can be p-channel transistors orCMOS switches.

A connection relation of the signal line driver circuit will bedescribed by using the circuit 502_1 as an example. First terminals ofthe transistors 503_1 to 503_k are connected to the wiring 505_1. Secondterminals of the transistors 503_1 to 503_k are connected to wirings S1to Sk, respectively. Gates of the transistors 503_1 to 503_k areconnected to wirings 504_1 to 504_k, respectively. For example, thefirst terminal of the transistor 503_1 is connected to the wiring 505_1,the second terminal of the transistor 503_1 is connected to the wiringS1, and the gate of the transistor 503_1 is connected to the wiring504_1.

The circuit 500 has a function of supplying a signal to the circuits502_1 to 502_N through the wirings 504_1 to 504_k and can function as ashift register or a decoder or the like. The signal is a digital signalin many cases and can function as a selection signal. In addition, thewirings 504_1 to 504_k can function as signal lines.

The circuit 501 has a function of outputting a signal to the circuits502_1 to 502_N and can function as a video signal generation circuit orthe like. For example, the circuit 501 supplies the signal to thecircuit 502_1 through the wiring 505_1. At the same time, the circuit501 supplies the signal to the circuit 502_2 through the wiring 505_2.The signal is an analog signal in many cases and can function as a videosignal. In addition, the wirings 505_1 to 505_N can function as signallines.

The circuits 502_1 to 502_k each have a function of selecting a wiringto which an output signal from the circuit 501 is output and canfunction as a selector circuit. For example, the circuit 502_1 has afunction of selecting one of the wirings S1 to Sk to output a signaloutput from the circuit 501 to the wiring 505_1.

The transistors 503_1 to 503_k have a function of controlling electricconduction state between the wiring 505_1 and the wirings S1 to Sk inaccordance with the output signal from the circuit 500 and function asswitches.

Next, operation of the signal line driver circuit shown in FIG. 26A isdescribed with reference to a timing chart in FIG. 26B. FIG. 26B showsexamples of a signal 514_1 input to the wiring 504_1, a signal 514_2input to the wiring 504_2, a signal 514_k input to the wiring 504_k, asignal 515_1 input to the wiring 505_1, and a signal 515_2 input to thewiring 5052.

Note that one operation period of the signal line driver circuitcorresponds to one gate selection period in a display device. One gateselection period is a period in which a pixel which belongs to one rowis selected and a video signal can be written to the pixel.

Note that one gate selection period is divided into a period T0 and aperiod T1 to a period Tk. The period T0 is a period for applying voltagefor precharge on pixels which belong to a selected row at the same timeand can be used as a precharge period. Each of the periods T1 to Tk is aperiod in which a video signal is written to pixels which belong to theselected row and can be used as a writing period.

Note that for simplicity, operation of the signal line driver circuit isdescribed by using operation of the circuit 502_1 as an example.

First, during the period T0, the circuit 500 outputs a signal in an Hlevel to the wirings 504_1 to 504_k. Then, the transistors 503_1 to503_k are turned on, whereby the wiring 505_1 and the wirings S1 to Skare brought into electrical conduction. At that time, the circuit 501supplies precharge voltage Vp to the wiring 505_1, so that the prechargevoltage Vp is output to the wirings S1 to Sk through the transistors503_1 to 503_k, respectively. Then, the precharge voltage Vp is writtento the pixels which belong to the selected row, whereby the pixels whichbelong to the selected row are precharged.

Next, during the period T1, the circuit 500 outputs a signal in an Hlevel to the wirings 504_1. Then, the transistor 503_1 is turned on,whereby the wiring 505_1 and the wiring S1 are brought into electricalconduction. Then, the wiring 505_1 and the wirings S2 to Sk are broughtout of electrical conduction. At that time, the circuit 501 supplies asignal Data (S1) to the wiring 505_1, so that the signal Data (S1) isoutput to the wiring S1 through the transistors 503_1. In this manner,the signal Data (S1) is written to, of the pixels connected to thewiring S1, the pixels which belong to the selected row.

Next, during the period T2, the circuit 500 outputs a signal in an Hlevel to the wirings 504_2. Then, the transistor 503_2 is turned on,whereby the wiring 505_2 and the wiring S2 are brought into electricalconduction. Then, the wiring 505_1 and the wirings S1 are brought out ofelectrical conduction while the wiring 505_1 and the wirings S3 to Skare kept out of electrical conduction. At that time, when the circuit501 outputs a signal Data (S2) to the wiring 505_1, the signal Data (S2)is output to the wiring S2 through the transistors 5032. In this manner,the signal Data (S2) is written to, of the pixels connected to thewiring S2, the pixels which belong to the selected row.

After that, since the circuit 500 sequentially outputs signals in an Hlevel to the wirings 504_1 to 504_k until the end of the period Tk, thecircuit 500 sequentially outputs the signal in the H level to thewirings 504_3 to 504_k from the period T3 to the period Tk, as in theperiod T1 and the period T2. Therefore, since the transistors 503_3 to503_k are sequentially turned on, the transistors 5031 to 503_k aresequentially turned on. Accordingly, a signal output from the circuit501 is sequentially output to the wirings S1 to Sk. In this manner, thesignal can be written to the pixels which belong to the selected row.

Since the signal line driver circuit in this embodiment includes thecircuit which functions as a selector, the number of signals or wiringscan be reduced. Alternatively, since voltage for precharging is writtento the pixel before a video signal is written to the pixel (duringperiod T0), a writing time of the video signal can be shortened.Accordingly, increase in the size of a display device and higherresolution of the display device can be achieved. However, thisembodiment is not limited to this. It is possible that the period T0 iseliminated, so that the pixel is not precharged.

Note that if k is too large a number, a writing time of the pixel isshortened, whereby writing of a video signal to the pixel is notcompleted in the writing time in some cases. Accordingly, it ispreferable that k≦6. It is more preferable that k≦3. It is furtherpreferable that k=2.

In specific, in the case where a color element of a pixel is dividedinto n (n is a natural number), it is possible that k=n. For example, inthe case where a color element of a pixel is divided into red (R), green(G), and blue (B), it is possible that k=3. In that case, one gateselection period is divided into a period T0, a period T1, a period T2,and a period T3. Then, a video signal can be written to the pixel of red(R), the pixel of green (G), and the pixel of blue (B) during the periodT1, the period T2, and the period T3, respectively. However, thisembodiment is not limited to this. The order of the period T1, theperiod T2, and the period T3 can be appropriately set.

In specific, in the case where a pixel is divided into n (n is a naturalnumber) sub-pixels, it is possible that k=n. For example, in the casewhere the pixel is divided into two sub-pixels, it is possible that k=2.In that case, one gate selection period is divided into the period T0,the period T1, and the period T2. Then, a video signal can be written toone of the two sub-pixels during the period T1, and a video signal canbe written to the other of the two sub-pixels during the period T2.

Note that since the driving frequencies of the circuit 500 and thecircuits 502_1 to 502_N are low in many cases, the circuit 500 and thecircuits 502_1 to 502_N can be formed over the same substrate as a pixelportion. In this manner, since the number of connections between thesubstrate over which the pixel portion is formed and an external circuitcan be reduced, increase in yield, improvement in reliability, or thelike can be achieved. Further, as shown in FIG. 23C, by also forming ascan line driver circuit over the same substrate as the pixel portion,the number of connections between the substrate over which the pixelportion is formed and the external circuit can be further reduced.

Note that the semiconductor device or shift register described inEmbodiments 1 to 4 can be used as the circuit 500. In that case, thepolarity of all transistors in the circuit 500 can be n-channel or thepolarity of all the transistors in the circuit 500 can be p-channel.Accordingly, reduction in the number of steps, increase in yield, orreduction in cost can be achieved.

Note that the polarity of not only all the transistors included in thecircuit 500 but also all transistors in the circuits 502_1 to 502_N canbe n-channel or all the transistors in the circuits 502_1 to 502_Np-channel. Therefore, in the case where the circuit 500 and the circuits502_1 to 502_N are formed over the same substrate as the pixel portion,reduction in the number of steps, increase in yield, or reduction incost can be achieved. In specific, by setting the polarity of alltransistors to be n-channel, non-single-crystal semiconductors,microcrystalline semiconductors, organic semiconductors, or oxidesemiconductors can be used as semiconductor layers of the transistors.This is because the driving frequencies of the circuit 500 and thecircuits 502_1 to 502_N are low in many cases.

Embodiment 8

In this embodiment, structures and operations of a pixel which can beapplied to a liquid crystal display device are described.

FIG. 27A illustrates an example of a pixel structure which can beapplied to the liquid crystal display device. A pixel 5080 includes atransistor 5081, a liquid crystal element 5082, and a capacitor 5083. Agate of the transistor 5081 is electrically connected to a wiring 5085.A first terminal of the transistor 5081 is electrically connected to awiring 5084. A second terminal of the transistor 5081 is electricallyconnected to a first terminal of the liquid crystal element 5082. Asecond terminal of the liquid crystal element 5082 is electricallyconnected to a wiring 5087. A first terminal of the capacitor 5083 iselectrically connected to the first terminal of the liquid crystalelement 5082. A second terminal of the capacitor 5083 is electricallyconnected to a wiring 5086.

The wiring 5084 can function as a signal line. The signal line is awiring for transmitting a signal voltage, which is input from theoutside of the pixel, to the pixel 5080. The wiring 5085 can function asa scan line. The scan line is a wiring for controlling on and off of thetransistor 5081. The wiring 5086 can function as a capacitor line. Thecapacitor line is a wiring for applying a predetermined voltage to thesecond terminal of the capacitor 5083. The transistor 5081 can functionas a switch. The capacitor 5083 can function as a storage capacitor. Thestorage capacitor is a capacitor with which the signal voltage continuesto be applied to the liquid crystal element 5082 even when the switch isoff. The wiring 5087 can function as a counter electrode. The counterelectrode is a wiring for applying a predetermined voltage to the secondterminal of the liquid crystal element 5082. Note that a function ofeach wiring is not limited thereto, and each wiring can have a varietyof functions. For example, by changing a voltage applied to thecapacitor line, a voltage applied to the liquid crystal element can beadjusted. Note that the transistor 5081 can be a p-channel transistor oran n-channel transistor because it merely functions as a switch.

FIG. 27B illustrates an example of a pixel structure which can beapplied to the liquid crystal display device. The example of the pixelstructure illustrated in FIG. 27B is the same as that in FIG. 27A exceptthat the wiring 5087 is eliminated and the second terminal of the liquidcrystal element 5082 and the second terminal of the capacitor 5083 areelectrically connected to each other. The example of the pixel structurein FIG. 27B can be particularly applied to the case of using ahorizontal electric field mode (including an IPS mode and FFS mode)liquid crystal element. This is because in the horizontal electric fieldmode liquid crystal element, the second terminal of the liquid crystalelement 5082 and the second terminal of the capacitor 5083 can be formedover one substrate, and thus it is easy to electrically connect thesecond terminal of the liquid crystal element 5082 and the secondterminal of the capacitor 5083. With the pixel structure in FIG. 10B,the wiring 5087 can be eliminated, whereby a manufacturing process canbe simplified, and manufacturing costs can be reduced.

A plurality of pixel structures illustrated in FIG. 27A or FIG. 27B canbe arranged in matrix. Accordingly, a display portion of a liquidcrystal display device is formed, and a variety of images can bedisplayed. FIG. 27C illustrates a circuit configuration in the casewhere a plurality of pixel structures illustrated in FIG. 27A arearranged in matrix. FIG. 27C is the circuit diagram illustrating fourpixels among a plurality of pixels included in the display portion. Apixel arranged in ith row and jth column (each of i and j is a naturalnumber) is represented as a pixel 5080_i, j, and a wiring 5084_i, awiring 5085_j, and a wiring 5086_j are electrically connected to thepixel 5080_i, j. Similarly, a wiring 5084_i+1, the wiring 5085_j, andthe wiring 5086_j are electrically connected to a pixel 5080_i+1, j.Similarly, the wiring 5084_i, a wiring 5085_j+1, and a wiring 5086_j+1are electrically connected to a pixel 5080_i, 1+1. Similarly, the wiring5084_i+1, the wiring 5085_j+1, and the wiring 5086_j+1 are electricallyconnected to a pixel 5080_i+1, j+1. Note that each wiring can be used incommon with a plurality of pixels in the same row or the same column. Inthe pixel structure illustrated in FIG. 27C, the wiring 5087 is acounter electrode, which is used by all the pixels in common; therefore,the wiring 5087 is not indicated by the natural number i or j. Further,since the pixel structure in FIG. 27B can also be used in thisembodiment, the wiring 5087 is not essential even in a structure wherethe wiring 5087 is described, and can be eliminated when another wiringfunctions as the wiring 5087, for example.

The pixel structure in FIG. 27C can be driven by a variety of drivingmethods. In particular, when the pixels are driven by a method calledalternating-current driving, degradation (burn-in) of the liquid crystalelement can be suppressed. FIG. 27D is a timing chart of voltagesapplied to each wiring in the pixel structure in FIG. 27C in the casewhere dot inversion driving which is a kind of alternating-currentdriving is performed. By the dot inversion driving, flickers seen whenthe alternating-current driving is performed can be suppressed. Notethat FIG. 27D shows a signal 5185_j which is input to the wiring 5085_j,a signal 5185_j+1 which is input to the wiring 5085_j+1, a signal 5184_iwhich is input to the wiring 5084_i, a signal 5184_j+1 which is input tothe wiring 5084_i+1, and voltage 5186 which is supplied to the wiring5086_j and the wiring 5086_j+1.

In the pixel structure in FIG. 27C, a switch in a pixel electricallyconnected to the wiring 5085_j is brought into a selection state (an onstate) in a jth gate selection period in one frame period, and into anon-selection state (an off state) in the other periods. Then, a (j+1)thgate selection period is provided after the jth gate selection period.By performing sequential scanning in such a manner, all the pixels aresequentially brought into a selection state within one frame period. Inthe timing chart of FIG. 27D, when a voltage is at high level, theswitch in the pixel is brought into a selection state; when a voltage isat low level, the switch is brought into a non-selection state. Notethat this is the case where the transistors in the pixels are n-channeltransistors. In the case of using p-channel transistors, the relationbetween the voltage and the selection state is opposite to that in thecase of using n-channel transistors.

In the timing chart illustrated in FIG. 27D, in the jth gate selectionperiod in a kth frame (k is a natural number), a positive signal voltageis applied to the wiring 5084_i used as a signal line, and a negativesignal voltage is applied to the wiring 5084_j+1. Then, in the (j+1)thgate selection period in the kth frame, a negative signal voltage isapplied to the wiring 5084_i, and a positive signal voltage is appliedto the wiring 5084_j+1. After that, signals whose polarity is reversedin each gate selection period are alternately supplied to the signalline. Thus, in the kth frame, the positive signal voltage is applied tothe pixels 5080_i, j and 5080_i+1, j+1, and the negative signal voltageis applied to the pixels 5080_i+1, j and 5080_i, j+1. Then, in a (k+1)thframe, a signal voltage whose polarity is opposite to that of the signalvoltage written in the kth frame is written to each pixel. Thus, in the(k+1)th frame, the positive signal voltage is applied to the pixels5080_i+1, j and 5080_i, j+1, and the negative signal voltage is appliedto the pixels 5080_i, j and 5080_i+1, j+1. In such a manner, the dotinversion driving is a driving method in which signal voltages whosepolarity is different between adjacent pixels are applied in one frameand the polarity of the voltage signal for the pixel is reversed in eachframe. By the dot inversion driving, flickers seen when the entire orpart of an image to be displayed is uniform can be suppressed whiledegradation of the liquid crystal element is suppressed. Note thatvoltages applied to all the wirings 5086 including the wirings 5086_jand 5086_j+1 can be a fixed voltage. Moreover, only the polarity of thesignal voltages for the wirings 5084 is shown in the timing chart, thesignal voltages can actually have a variety of values in the polarityshown. Here, the case where the polarity is reversed per dot (per pixel)is described; however, this embodiment is not limited thereto, and thepolarity can be reversed per a plurality of pixels. For example, thepolarity of signal voltages to be written is reversed per two gateselection periods, whereby power consumed by writing the signal voltagescan be reduced. Alternatively, the polarity may be reversed per column(source line inversion) or per row (gate line inversion).

Note that a fixed voltage may be applied to the second terminal of thecapacitor 5083 in the pixel 5080 in one frame period. Since a voltageapplied to the wiring 5085 used as a scan line is at low level in mostof one frame period, which means that a substantially constant voltageis applied to the wiring 5085; therefore, the second terminal of thecapacitor 5083 in the pixel 5080 may be connected to the wiring 5085.FIG. 27E illustrates an example of a pixel structure which can beapplied to the liquid crystal display device. Compared to the pixelstructure in FIG. 27C, a feature of the pixel structure in FIG. 27E isthat the wiring 5086 is eliminated and the second terminal of thecapacitor 5083 in the pixel 5080 and the wiring 5085 in the previous roware electrically connected to each other. Specifically, in the rangeillustrated in FIG. 27E, the second terminals of the capacitors 5083 inthe pixels 5080_j, j+1 and 5080_i+1, j+1 are electrically connected tothe wiring 5085_j. By electrically connecting the second terminal of thecapacitor 5083 in the pixel 5080 and the wiring 5085 in the previous rowin such a manner, the wiring 5086 can be eliminated, so that theaperture ratio of the pixel can be increased. Note that the secondterminal of the capacitor 5083 may be connected to the wiring 5085 inanother row instead of in the previous row. Further, the pixel structurein FIG. 27E can be driven by a similar driving method to that in thepixel structure in FIG. 27C.

Note that a voltage applied to the wiring 5084 used as a signal line canbe made lower by using the capacitor 5083 and the wiring electricallyconnected to the second terminal of the capacitor 5083. A pixelstructure and a driving method in that case will be described withreference to FIGS. 27F and 27G Compared to the pixel structure in FIG.27A, a feature of the pixel structure in FIG. 27F is that two wirings5086 are provided per pixel row, and in adjacent pixels, one wiring iselectrically connected to every other second terminal of the capacitors5083 and the other wiring is electrically connected to the remainingevery other second terminal of the capacitors 5083. Two wirings 5086 arereferred to as a wiring 5086-1 and a wiring 5086-2. Specifically, in therange illustrated in FIG. 27F, the second terminal of the capacitor 5083in the pixel 5080_i, j is electrically connected to a wiring 5086-1_j;the second terminal of the capacitor 5083 in the pixel 5080_i+1, j iselectrically connected to a wiring 5086-2_j; the second terminal of thecapacitor 5083 in the pixel 5080_i, j+1 is electrically connected to awiring 5086-2_j+1; and the second terminal of the capacitor 5083 in thepixel 5080_i+1, j+1 is electrically connected to a wiring 5086-1_j+1.Note that FIG. 27G shows the signal 5185_j which is input to the wiring5085_j, the signal 5185_j+1 which is input to the wiring 5085_j+1, thesignal 5184_i which is input to the wiring 5084_i, the signal 5184_i+1which is input to the wiring 5084_i+1, a signal 5186-1_j which is inputto the wiring 5086-1_j, a signal 5186-2_j which is input to the wiring5086-2_j, a signal 5186-1_j+1 which is input to the wiring 5086-1_j+1,and a signal 5186-2_j+1 which is input to the wiring 5086-2_j+1.

For example, when a positive signal voltage is written to the pixel5080_i, j in the kth frame as illustrated in FIG. 27G; the wiring5086-1_j becomes low level, and is changed to high level after the jthgate selection period. Then, the wiring 5086-1_j is kept at high levelin one frame period, and after a negative signal voltage is written inthe jth gate selection period in the (k+1)th frame, the wiring 5086-1_jis changed to high level. In such a manner, a voltage of the wiringwhich is electrically connected to the second terminal of the capacitor5083 is changed to the positive direction after a positive signalvoltage is written to the pixel, whereby a voltage applied to the liquidcrystal element can be changed to the positive direction by apredetermined amount. That is, a signal voltage written to the pixel canbe reduced accordingly, so that power consumed by signal writing can bereduced. Note that when a negative signal voltage is written in the jthgate selection period, a voltage of the wiring which is electricallyconnected to the second terminal of the capacitor 5083 is changed to thenegative direction after a negative signal voltage is written to thepixel. Accordingly, a voltage applied to the liquid crystal element canbe changed to the negative direction by a predetermined amount, and thesignal voltage written to the pixel can be reduced as in the case of thepositive polarity. In other words, as for the wiring which iselectrically connected to the second terminal of the capacitor 5083,different wirings are preferably used for a pixel to which a positivesignal voltage is applied and a pixel to which a negative signal voltageis applied in the same row in one frame. FIG. 27F illustrates theexample in which the wiring 5086-1 is electrically connected to thepixel to which a positive signal voltage is applied in the kth frame,and the wiring 5086-2 is electrically connected to the pixel to which anegative signal voltage is applied in the kth frame. Note that this isjust an example, and for example, in the case of using a driving methodin which pixels to which a positive signal voltage is applied and pixelsto which a negative signal voltage is applied are arranged every twopixels, the wirings 5086-1 and 5086-2 are preferably electricallyconnected to every alternate two pixels accordingly. Furthermore, in thecase where signal voltages of the same polarity are written in all thepixels in one row (gate line inversion), one wiring 5086 may be providedper row. In other words, in the pixel structure in FIG. 27C, the drivingmethod where a signal voltage written to a pixel is reduced as describedwith reference to FIGS. 27F and 27G can be used.

Next, a pixel structure and a driving method which are preferablyemployed particularly in the case where a liquid crystal element employsa vertical alignment (VA) mode typified by an MVA mode and a PVA mode.The VA mode has advantages such as no rubbing step in manufacture,little light leakage at the time of black display, and low drivingvoltage, but has a problem in that the image quality is degraded (theviewing angle is narrower) when a screen is seen from an oblique angle.In order to increase the viewing angle in the VA mode, a pixel structurewhere one pixel includes a plurality of subpixels as illustrated inFIGS. 28A and 28B is effective. Pixel structures illustrated in FIGS.28A and 28B are examples of the case where the pixel 5080 includes twosubpixels (a subpixel 5080-1 and a subpixel 5080-2). Note that thenumber of subpixels in one pixel is not limited to two and can be othernumbers. The viewing angle can be further increased as the number ofsubpixels is increased. A plurality of subpixels can have the samecircuit configuration; here, all the subpixels have the circuitconfiguration illustrated in FIG. 27A. The first subpixel 5080-1includes a transistor 5081-1, a liquid crystal element 5082-1, and acapacitor 5083-1. The connection relation is the same as that in thecircuit configuration in FIG. 27A. Similarly, the second subpixel 5080-2includes a transistor 5081-2, a liquid crystal element 5082-2, and acapacitor 5083-2. The connection relation is the same as that in thecircuit configuration in FIG. 27A.

The pixel structure in FIG. 28A includes, for two subpixels forming onepixel, two wirings 5085 (a wiring 5085-1 and a wiring 5085-2) used asscan lines, one wiring 5084 used as a signal line, and one wiring 5086used as a capacitor line. When the signal line and the capacitor lineare shared with two subpixels in such a manner, the aperture ratio canbe increased. Further, since a signal line driver circuit can besimplified, manufacturing costs can be reduced. Moreover, since thenumber of connections between a liquid crystal panel and a drivercircuit IC can be reduced, the yield can be increased. The pixelstructure in FIG. 28B includes, for two subpixels forming one pixel, onewiring 5085 used as a scan line, two wirings 5084 (a wiring 5084-1 and awiring 5084-2) used as signal lines, and one wiring 5086 used as acapacitor line. When the scan line and the capacitor line are sharedwith two subpixels in such a manner, the aperture ratio can beincreased. Further, since the total number of scan lines can be reduced,one gate line selection period can be sufficiently long even in ahigh-definition liquid crystal panel, and an appropriate signal voltagecan be written in each pixel.

FIGS. 28C and 28D illustrate an example in which the liquid crystalelement in the pixel structure in FIG. 28B is replaced with the shape ofa pixel electrode and electrical connections of each element areschematically shown. In FIGS. 28C and 28D, an electrode 5088-1represents a first pixel electrode, and an electrode 5088-2 represents asecond pixel electrode. In FIG. 28C, the first pixel electrode 5088-1corresponds to a first terminal of the liquid crystal element 5082-1 inFIG. 28B, and the second pixel electrode 5088-2 corresponds to a firstterminal of the liquid crystal element 5082-2 in FIG. 28B. That is, thefirst pixel electrode 5088-1 is electrically connected to one of asource and a drain of the transistor 5081-1, and the second pixelelectrode 5088-2 is electrically connected to one of a source and adrain of the transistor 5081-2. In FIG. 28D, the connection relationbetween the pixel electrode and the transistor is opposite to that inFIG. 28C. That is, the first pixel electrode 5088-1 is electricallyconnected to one of the source and the drain of the transistor 5081-2,and the second pixel electrode 5088-2 is electrically connected to oneof the source and the drain of the transistor 5081-1.

By arranging a plurality of pixel structures as illustrated in FIG. 28Cor FIG. 28D in matrix, an extraordinary effect can be obtained. FIGS.28E and 28F illustrate an example of such a pixel structure and drivingmethod. In the pixel structure in FIG. 28E, a portion corresponding tothe pixels 5080_i, j and 5080_i+1, j+1 has the structure illustrated inFIG. 28C, and a portion corresponding to the pixels 5080_i+1, j and5080_i, j+1 has the structure illustrated in FIG. 28D. When thisstructure is driven as shown in the timing chart of FIG. 28F, a positivesignal voltage is written to the first pixel electrode in the pixel5080_i, j and the second pixel electrode in the pixel 5080_i+1, j, and anegative signal voltage is written to the second pixel electrode in thepixel 5080_i, j and the first pixel electrode in the pixel 5080_i+1, j.Then, in the (j+1)th gate selection period in the kth frame, a positivesignal voltage is written to the second pixel electrode in the pixel5080_i, j+1 and the first pixel electrode in the pixel 5080_i+1, j+1,and a negative signal voltage is written to the first pixel electrode inthe pixel 5080_j, j+1 and the second pixel electrode in the pixel5080_i+1, j+1. In the (k+1)th frame, the polarity of the signal voltageis reversed in each pixel. Accordingly, the polarity of the voltageapplied to the signal line can be the same in one frame period whiledriving corresponding to dot inversion driving is realized in the pixelstructure including subpixels, whereby power consumed by writing thesignal voltages to the pixels can be drastically reduced. Note thatvoltages applied to all the wirings 5086 including the wirings 5086_jand 5086_j+1 can be a fixed voltage. Note that FIG. 27F shows the signal5185_j which is input to the wiring 5085_j, the signal 5185_j+1 which isinput to the wiring 5085_j+1, the signal 5184-1_i which is input to thewiring 5084-1_i, the signal 5184-2_i which is input to the wiring5084-2_i, a signal 5184-1_i+1 which is input to the wiring 5084-1_j+1, asignal 5184-2_i+1 which is input to the wiring 5084-2_i+1, and thevoltage 5186 which is supplied to the wiring 5086_j and the wiring5086_j+1.

Further, by a pixel structure and a driving method illustrated in FIGS.28G and 28H, the level of the signal voltage written to a pixel can bereduced. In the structure, a plurality of subpixels included in eachpixel are electrically connected to respective capacitor lines. That is,according to the pixel structure and the driving method illustrated inFIGS. 28G and 28H, one capacitor line is shared with subpixels in onerow, to which signal voltages of the same polarity are written in oneframe; and subpixels to which signal voltages of the differentpolarities are written in one frame use different capacitor lines in onerow. Then, when writing in each row is finished, voltages of thecapacitor lines are changed to the positive direction in the subpixelsto which a positive signal voltage is written, and changed to thenegative direction in the subpixels to which a negative signal voltageis written; thus, the level of the signal voltage written to the pixelcan be reduced. Specifically, two wirings 5086 (the wirings 5086-1 and5086-2) used as capacitor lines are provided per row. The first pixelelectrode in the pixel 5080_i, j and the wiring 5086-1_j areelectrically connected through the capacitor. The second pixel electrodein the pixel 5080_i, j and the wiring 5086-2_j are electricallyconnected through the capacitor. The first pixel electrode in the pixel5080_i+1, j and the wiring 5086-2_j are electrically connected throughthe capacitor. The second pixel electrode in the pixel 5080_i+1, j andthe wiring 5086-1_j are electrically connected through the capacitor.The first pixel electrode in the pixel 5080_i, j+1 and the wiring5086-2_j+1 are electrically connected through the capacitor. The secondpixel electrode in the pixel 5080_i, j+1 and the wiring 5086-1_j+1 areelectrically connected through the capacitor. The first pixel electrodein the pixel 5080_i+1, j+1 and the wiring 5086-1_j+1 are electricallyconnected through the capacitor. The second pixel electrode in the pixel5080_i+1, j+1 and the wiring 5086-2_j+1 are electrically connectedthrough the capacitor. Note that this is just an example, and forexample, in the case of using a driving method in which pixels to whicha positive signal voltage is applied and pixels to which a negativesignal voltage is applied are arranged every two pixels, the wirings5086-1 and 5086-2 are preferably electrically connected to everyalternate two pixels accordingly. Furthermore, in the case where signalvoltages of the same polarity are written in all the pixels in one row(gate line inversion), one wiring 5086 may be provided per row. In otherwords, in the pixel structure in FIG. 28E, the driving method where asignal voltage written to a pixel is reduced as described with referenceto FIGS. 28G and 28H can be used. Note that FIG. 28H shows the signal5185_j which is input to the wiring 5085_j, the signal 5185_j+1 which isinput to the wiring 5085_j+1, the signal 5184-1_i which is input to thewiring 5084-1_i, the signal 5184-2_i which is input to the wiring5084-2_i, the signal 5184-1_i+1 which is input to the wiring 5084-1_i+1,the signal 5184-2_i+1 which is input to the wiring 5084-2_i+1, thesignal 5186-1_j which is input to the wiring 5086-1_j, the signal5186-2_j which is input to the wiring 5086-2_j, the signal 5186-1_j+1which is input to the wiring 5086-1_j+1, and the signal 5186-2_j+1 whichis input to the wiring 5086-2_j+1.

By a combination of the pixel in this embodiment and the semiconductordevice, shift register, or display device in Embodiments 1 to 7, avariety of advantages can be obtained. For example, in the case wherethe pixel with a sub-pixel structure is used, since the number ofsignals required for driving the display device is increased, the numberof gate lines or source lines is increased in some cases. As a result,the number of connections between a substrate over which a pixel portionis formed and an external circuit is largely increased in some cases.However, even if the number of gate lines is increased, a scan linedriver circuit can be formed over the same substrate as the pixelportion as shown in Embodiment 5. Accordingly, the pixel with thesub-pixel structure can be used without largely increasing the number ofconnections between the substrate over which the pixel portion is formedand the external circuit. Alternatively, even if the number of sourcelines is increased, the number of source lines can be decreased by usingthe signal line driver circuit in Embodiment 7. Therefore, the pixelwith the sub-pixel structure can be used without largely increasing thenumber of connections between the substrate over which the pixel portionis formed and the external circuit.

Alternatively, in the case where a signal is input to a capacitor line,the number of connections between the substrate over which the pixelportion is formed and the external circuit is largely increased in somecases. Accordingly, the signal can be supplied to the capacitor line byusing the semiconductor device or shift register in Embodiments 1 to 4.In addition, the semiconductor device or shift register in Embodiments 1to 4 can be formed over the same substrate as the pixel portion.Therefore, the signal can be input to the capacitor line without largelyincreasing the number of connections between the substrate over whichthe pixel portion is formed and the external circuit.

Alternatively, in the case where alternate-current driving is employed,a writing time of a video signal to the pixel becomes long. As a result,enough writing time of the video signal to the pixel cannot be obtainedin some cases. Similarly, in the case where the pixel with the sub-pixelstructure is used, the writing time of the video signal to the pixel isshortened. As a result, enough writing time of the video signal to thepixel cannot be obtained in some cases. By using the signal line drivercircuit in Embodiment 7, the video signal can be written to the pixel.In that case, since voltage for precharge is written to the pixel beforethe video signal is written to the pixel, the video signal can bewritten to the pixel in a short time. Alternatively, as shown in FIG. 24and FIGS. 25A and 25B, by overlapping a period in which one row isselected and a period in which a different row is selected with eachother, a video signal in a different row can be used as the voltage forprecharging.

Note that by a combination of the driving method of the pixel in thisembodiment and the driving method shown in FIG. 24 and FIGS. 25A and25B, the writing time of the video signal to the pixel can be shortened.This is described in detail with reference to a timing chart in FIG. 29Aand a pixel structure in FIG. 27C. A positive video signal is input tothe wiring 5084_i and a negative video signal is input to the wiring5084_i+1 in the kth frame. In addition, a negative video signal is inputto the wiring 5084_i and a positive video signal is input to the wiring5084_i+1 in the (k+1)th frame. In the (k+1)th frame, so-called sourceline inversion driving is performed. Moreover, for example, the latterhalf of a period in which an H signal is input to the wiring 5085_j andthe first half of a period in which an H signal is input to the wiring5085_j+1 overlap with each other. Further, a negative video signal iswritten to and held in the pixels 5080_i and 5080 j+1 in the (k−1)thframe. A positive video signal is written to and held in the pixels5080_j+1 and 5080_j+1. Note that FIG. 29A shows the signal 5185_j whichis input to the wiring 5085_j, the signal 5185_j+1 which is input to thewiring 5085_j+1, the signal 5184_i which is input to the wiring 5084_i,and the signal 5184_i+1 which is input to the wiring 5084_i+1.

First, in the kth frame, a positive video signal is written to thepixels 5080_i and 5080 j and a negative video signal is written to thepixels 5080_j+1 and 5080_j during a period when the period in which theH signal is input to the wiring 5085_j and the period in which the Hsignal is input to the wiring 5085_j+1 overlap with each other.

At that time, the positive video signal is also written to the pixels5080_i and 5080_j+1 and the negative video signal is also written to thepixels 5080_i+1 and 5080_j+1. In this manner, pixels in a (j+1)th roware precharged by using the video signal written to pixels in a jth row.After that, in the kth frame, a positive video signal is written to thepixels 5080_i and 5080_j+1 and a negative video signal is written to thepixels 5080_i+1 and 5080_j+1 during the latter half of the period inwhich the H signal is input to the wiring 5080_j+1. It is needless tosay that the positive video signal is written to the pixel 5080_i and apixel 5080_j+2, whereby the pixels 5080_i and 5080 j+2 are precharged.Similarly, the negative video signal is written to the pixels 5080_i+1and 5080_j+2, whereby the pixels 5080_i+1 and 5080_j+2 are precharged.In this manner, by precharging the pixels in the (j+1)th row by usingthe video signal written to the pixels in the jth row, a writing time ofthe video signal to the pixels in the (j+1)th row can be shortened.

Note that by a combination of the driving method in FIG. 29A and thepixel structure in FIG. 29B, dot inversion driving can be realized. Inthe pixel structure in FIG. 29B, the pixels 5080_i and 5080_j areconnected to the wiring 5084_i. On the other hand, the pixels 5080_i and5080_j+1 are connected to the wiring 5084_i+1. That is, each of pixelsin an ith column is alternately connected to the wiring 5084_i or thewiring 5084_i+1 with respect to one row. In this manner, since apositive video signal or a negative video signal is alternately writtento each of the pixels in the ith column, the dot inversion driving canbe realized. However, this embodiment is not limited to this. Each ofthe pixels in the ith column can be alternately connected to the wiring5084_i or the wiring 5084_i+1 with respect to a plurality of rows (e.g.,two rows or three rows).

Embodiment 9

In this embodiment, examples of structures of transistors are describedwith reference to FIGS. 32A, 32B, and 32C.

FIG. 32A illustrates an example of a structure of a top-gate transistor.FIG. 32B illustrates an example of a structure of a bottom-gatetransistor. FIG. 32C illustrates an example of a structure of atransistor formed using a semiconductor substrate.

FIG. 32A illustrates a substrate 5260; an insulating layer 5261 formedover the substrate 5260; a semiconductor layer 5262 which is formed overthe insulating layer 5261 and is provided with a region 5262 a, a region5262 b, a region 5262 c, a region 5262 d, and a region 5262 e; aninsulating layer 5263 formed so as to cover the semiconductor layer5262; a conductive layer 5264 formed over the semiconductor layer 5262and the insulating layer 5263; an insulating layer 5265 which is formedover the insulating layer 5263 and the conductive layer 5264 and isprovided with openings; a conductive layer 5266 which is formed over theinsulating layer 5265 and in the openings formed in the insulating layer5265; an insulating layer 5267 which is formed over the conductive layer5266 and the insulating layer 5265 and is provided with an opening; aconductive layer 5268 which is formed over the insulating layer 5267 andin the opening formed in the insulating layer 5267; an insulating layer5269 which is formed over the insulating layer 5267 and the conductivelayer 5268 and is provided with the opening; a light-emitting layer 5270which is formed over the insulating layer 5269 and in the opening formedin the insulating layer 5269; and a conductive layer 5271 formed overthe insulating layer 5269 and the light-emitting layer 5270.

FIG. 32B illustrates a substrate 5300; a conductive layer 5301 formedover the substrate 5300; an insulating layer 5302 formed so as to coverthe conductive layer 5301; a semiconductor layer 5303 a formed over theconductive layer 5301 and the insulating layer 5302; a semiconductorlayer 5303 b formed over the semiconductor layer 5303 a; a conductivelayer 5304 formed over the semiconductor layer 5303 b and the insulatinglayer 5302; an insulating layer 5305 which is formed over the insulatinglayer 5302 and the conductive layer 5304 and is provided with anopening; a conductive layer 5306 which is formed over the insulatinglayer 5305 and in the opening formed in the insulating layer 5305; aliquid crystal layer 5307 formed over the insulating layer 5305 and theconductive layer 5306; and a conductive layer 5308 formed over theliquid crystal layer 5307.

FIG. 32C illustrates a semiconductor substrate 5352 including a region5353 and a region 5355; an insulating layer 5356 formed over thesemiconductor substrate 5352; an insulating layer 5354 formed over thesemiconductor substrate 5352; a conductive layer 5357 formed over theinsulating layer 5356; an insulating layer 5358 which is formed over theinsulating layer 5354, the insulating layer 5356, and the conductivelayer 5357 and is provided with openings; and a conductive layer 5359which is formed over the insulating layer 5358 and in the openingsformed in the insulating layer 5358. Thus, a transistor is formed ineach of a region 5350 and a region 5351.

The insulating layer 5261 can function as a base film. The insulatinglayer 5354 functions as an element isolation layer (e.g., a field oxidefilm). Each of the insulating layer 5263, the insulating layer 5302, andthe insulating layer 5356 can function as a gate insulating film. Eachof the conductive layer 5264, the conductive layer 5301, and theconductive layer 5357 can function as a gate electrode. Each of theinsulating layer 5265, the insulating layer 5267, the insulating layer5305, and the insulating layer 5358 can function as an interlayer filmor a planarization film. Each of the conductive layer 5266, theconductive layer 5304, and the conductive layer 5359 can function as awiring, an electrode of a transistor, an electrode of a capacitor, orthe like. Each of the conductive layer 5268 and the conductive layer5306 can function as a pixel electrode, a reflective electrode, or thelike. The insulating layer 5269 can function as a bank. Each of theconductive layer 5271 and the conductive layer 5308 can function as acounter electrode, a common electrode, or the like.

As each of the substrate 5260 and the substrate 5300, a glass substrate,a quartz substrate, a silicon substrate, a metal substrate, a stainlesssteel substrate, a flexible substrate, or the like can be used, forexample. As a glass substrate, a barium borosilicate glass substrate, analuminoborosilicate glass substrate, or the like can be used, forexample. For a flexible substrate, a flexible synthetic resin such asplastics typified by polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), and polyether sulfone (PES), or acrylic can be used,for example. Alternatively, an attachment film (formed usingpolypropylene, polyester, vinyl, polyvinyl fluoride, polyvinyl chloride,or the like), paper of a fibrous material, a base material film (formedusing polyester, polyamide, an inorganic vapor deposition film, paper,or the like), or the like can be used.

As the semiconductor substrate 5352, for example, a single crystalsilicon substrate having n-type or p-type conductivity can be used. Notethat this embodiment is not limited to this, and a substrate which issimilar to the substrate 5260 can be used. For example, the region 5353is a region where an impurity is added to the semiconductor substrate5352 and functions as a well. For example, in the case where thesemiconductor substrate 5352 has p-type conductivity, the region 5353has n-type conductivity and functions as an n-well. On the other hand,in the case where the semiconductor substrate 5352 has n-typeconductivity, the region 5353 has p-type conductivity and functions as ap-well. For example, the region 5355 is a region where an impurity isadded to the semiconductor substrate 5352 and functions as a sourceregion or a drain region. Note that an LDD region can be formed in thesemiconductor substrate 5352.

For the insulating layer 5261, a single-layer structure or a layeredstructure of an insulating film containing oxygen or nitrogen, such assilicon oxide (SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride(SiO_(x)N_(y)) (x>y), or silicon nitride oxide (SiN_(x)O_(y)) (x>y) canbe used, for example. In an example in the case where the insulatingfilm 5261 has a two-layer structure, a silicon nitride film and asilicon oxide film can be formed as a first insulating film and a secondinsulating film, respectively. In an example in the case where theinsulating film 5261 has a three-layer structure, a silicon oxide film,a silicon nitride film, and a silicon oxide film can be formed as afirst insulating film, a second insulating film, and a third insulatingfilm, respectively.

As an example of each of the semiconductor layer 5262, the semiconductorlayer 5303 a, and the semiconductor layer 5303 b, a single layerstructure or layered structure of an amorphous semiconductor,microcrystalline (microcrystal) semiconductor, a polycrystallinesemiconductor, a single crystal semiconductor, an oxide semiconductor(e.g., zinc oxide (ZnO) or IGZO (InGaZnO), or compound semiconductor(e.g., gallium arsenide (GaAs)) can be given.

Note that for example, the region 5262 a is an intrinsic region where animpurity is not added to the semiconductor layer 5262 and functions as achannel region. However, a slight amount of impurities can be added tothe region 5262 a. The concentration of the impurity added to the region5262 a is preferably lower than the concentration of an impurity addedto the region 5262 b, the region 5262 c, the region 5262 d, or theregion 5262 e. Each of the region 5262 b and the region 5262 d is aregion to which an impurity is added at low concentration and functionsas an LDD (lightly doped drain) region. Note that the region 5262 b andthe region 5262 d can be eliminated. Each of the region 5262 c and theregion 5262 e is a region to which an impurity is added at highconcentration and functions as a source region or a drain region.

Note that in the case where the semiconductor layer 5262 is used for atransistor, the conductivity type of the region 5262 c and theconductivity type of the region 5262 e are preferably the same.

Note that the semiconductor layer 5303 b is a semiconductor layer towhich phosphorus or the like is added as an impurity element and hasn-type conductivity.

Note that in the case where an oxide semiconductor or a compoundsemiconductor is used for the semiconductor layer 5303 a, thesemiconductor layer 5303 b can be eliminated.

For each of the insulating layer 5263, the insulating layer 5302, andthe insulating layer 5356, a single-layer structure or a layeredstructure of an insulating film containing oxygen or nitrogen, such assilicon oxide (SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride(SiO_(x)N_(y)) (x>y), or silicon nitride oxide (SiN_(x)O_(y)) (x>y) canbe used, for example.

As each of the conductive layer 5264, the conductive layer 5266, theconductive layer 5268, the conductive layer 5271, the conductive layer5301, the conductive layer 5304, the conductive layer 5306, theconductive layer 5308, the conductive layer 5357, and the conductivelayer 5359, for example, a conductive film having a single-layerstructure or a layered structure, or the like can be used. For example,for the conductive film, a single-layer film containing one elementselected from the group consisting of aluminum (Al), tantalum (Ta),titanium (Ti), molybdenum (Mo), tungsten (W), neodymium (Nd), chromium(Cr), nickel (Ni), platinum (Pt), gold (Au), silver (Ag), copper (Cu),manganese (Mn), cobalt (Co), niobium (Nb), silicon (Si), iron (Fe),palladium (Pd), carbon (C), scandium (Sc), zinc (Zn), phosphorus (P),boron (B), arsenic (As), gallium (Ga), indium (In), tin (Sn), and oxygen(O); a compound containing one or more elements selected from the abovegroup; or the like can be used. For example, the compound is an alloycontaining one or more elements selected from the above group (e.g., analloy material such as indium tin oxide (ITO), indium zinc oxide (IZO),indium tin oxide containing silicon oxide (ITSO), zinc oxide (ZnO), tinoxide (SnO), cadmium tin oxide (CTO), aluminum-neodymium (Al—Nd),magnesium-silver (Mg—Ag), molybdenum-niobium (Mo—Nb),molybdenum-tungsten (Mo—W), or molybdenum-tantalum (Mo—Ta)); a compoundcontaining nitrogen and one or more elements selected from the abovegroup (e.g., a nitride film containing titanium nitride, tantalumnitride, molybdenum nitride, or the like); or a compound containingsilicon and one or more elements selected from the above group (e.g., asilicide film containing tungsten silicide, titanium silicide, nickelsilicide, aluminum silicon, or molybdenum silicon); or the like.Alternatively, a nanotube material such as a carbon nanotube, an organicnanotube, an inorganic nanotube, or a metal nanotube can be used.

Note that silicon (Si) can contain an n-type impurity (e.g., phosphorus)or a p-type impurity (e.g., boron).

Note that in the case where copper is used for the conductive layer, alayered structure is preferably used in order to improve adhesion.

Note that for a conductive layer which is in contact with an oxidesemiconductor or silicon, molybdenum or titanium is preferably used.

Note that by using an alloy material containing neodymium and aluminumfor the conductive layer, aluminum does not easily cause hillocks.

Note that in the case where a semiconductor material such as silicon isused for the conductive layer, the semiconductor material such assilicon can be formed at the same time as a semiconductor layer of atransistor.

Note that since ITO, IZO, ITSO, ZnO, Si, SnO, CTO, a carbon nanotube, orthe like has light-transmitting properties, such a material can be usedfor a portion through which light passes, such as a pixel electrode, acounter electrode, or a common electrode.

Note that by using a layered structure containing a low-resistancematerial (e.g., aluminum), wiring resistance can be lowered.

Note that by using a layered structure where a low heat-resistancematerial (e.g., aluminum) is interposed between high heat-resistancematerials (e.g., molybdenum, titanium, or neodymium), advantages of thelow heat-resistance material can be effectively utilized and heatresistance of a wiring, an electrode, or the like can be increased.

Note that a material whose properties are changed by reaction with adifferent material can be interposed between or covered with materialswhich do not easily react with the different material. For example, inthe case where ITO and aluminum are connected to each other, titanium,molybdenum, or an alloy of neodymium can be interposed between ITO andaluminum. For example, in the case where silicon and aluminum areconnected to each other, titanium, molybdenum, or an alloy of neodymiumcan be interposed between silicon and aluminum. Note that such amaterial can be used for a wiring, an electrode, a conductive layer, aconductive film, a terminal, a via, a plug, or the like.

Note that in the case where the above-described conductive film isformed to have a layered structure, for example, a structure in which Alis sandwiched between Mo, Ti, or the like is preferable. Thus, theresistance of Al to heat or chemical reaction can be improved.

For each of the insulating layer 5265, the insulating layer 5267, theinsulating layer 5269, the insulating layer 5305, and the insulatinglayer 5358, an insulating film having a single-layer structure or alayered structure, or the like can be used, for example. For example, asthe insulating film, an insulating film containing oxygen or nitrogen,such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)), siliconoxynitride (SiO_(x)N_(y)) (x>y), or silicon nitride oxide (SiN_(x)O_(y))(x>y); a film containing carbon such as diamond-like carbon (DLC); anorganic material such as a siloxane resin, epoxy, polyimide, polyamide,polyvinyl phenol, benzocyclobutene, or acrylic; or the like can be used.

For the light-emitting layer 5270, an organic EL element, an inorganicEL element, or the like can be used, for example. For the organic ELelement, for example, a single-layer structure or a layered structure ofa hole injection layer formed using a hole injection material, a holetransport layer formed using a hole transport material, a light-emittinglayer formed using a light-emitting material, an electron transportlayer formed using an electron transport material, an electron injectionlayer formed using an electron injection material, or a layer in which aplurality of these materials are mixed can be used.

Note that an insulating layer which functions as an alignment film, aninsulating layer which functions as a protrusion portion, or the likecan be formed over the insulating layer 5305 and the conductive layer5306.

Note that an insulating layer or the like which functions as a colorfilter, a black matrix, or a protrusion portion can be formed over theconductive layer 5308. An insulating layer which functions as analignment film can be formed below the conductive layer 5308.

Note that the insulating layer 5269, the light-emitting layer 5270, andthe conductive layer 5271 can be eliminated in the cross-sectionalstructure in FIG. 32A, and the liquid crystal layer 5307 and theconductive layer 5308 which are illustrated in FIG. 32B can be formedover the insulating layer 5267 and the conductive layer 5268.

Note that the liquid crystal layer 5307 and the conductive layer 5308can be eliminated in the cross-sectional structure in FIG. 32B, and theinsulating layer 5269, the light-emitting layer 5270, and the conductivelayer 5271 which are illustrated in FIG. 32A can be formed over theinsulating layer 5305 and the conductive layer 5306.

Note that in the cross-sectional structure in FIG. 32C, the insulatinglayer 5269, the light-emitting layer 5270, and the conductive layer 5271which are illustrated in FIG. 32A can be formed over the insulatinglayer 5358 and the conductive layer 5359.

Alternatively, the liquid crystal layer 5307 and the conductive layer5308 which are illustrated in FIG. 32B can be formed over the insulatinglayer 5267 and the conductive layer 5268.

The shift register in this embodiment can be used for the semiconductordevice, shift register, or display device in Embodiments 1 to 8. Inspecific, in the case where a non-single crystal semiconductor,microcrystalline semiconductor, an organic semiconductor, an oxidesemiconductor, or the like is used as a semiconductor layer of thetransistor in FIG. 32B, the transistor deteriorates in some cases. Insuch a case too, deterioration of the transistor can be suppressed inthe semiconductor, shift register, or display device in Embodiments 1 to8.

Embodiment 10

In this embodiment, examples of electronic devices are described.

FIGS. 33A to 33H and FIGS. 34A to 34D are diagrams illustratingelectronic devices. These electronic devices can each include a housing5000, a display portion 5001, a speaker 5003, an LED lamp 5004, anoperation key 5005, a connecting terminal 5006, a sensor 5007 (a sensorhaving a function of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone 5008, and the like.

FIG. 33A is a mobile computer which can include a switch 5009, aninfrared rays port 5010, and the like in addition to the above-describedobjects. FIG. 33B illustrates a portable image reproducing device (e.g.,a DVD reproducing device) provided with a memory medium, which caninclude a second display portion 5002, a memory medium reading portion5011, and the like in addition to the above objects. FIG. 33Cillustrates a goggle-type display which can include the second displayportion 5002, a supporting portion 5012, an earphone 5013, and the likein addition to the above objects. FIG. 33D illustrates a portable gamemachine which can include the memory medium reading portion 5011 and thelike in addition to the above objects. FIG. 33E is a projector which caninclude a light source 5033, a projecting lens 5034, and the like inaddition to the above-described objects. FIG. 33F is a portable gamemachine which can include a second display portion 5002, a memory mediumreading portion 5011, and the like in addition to the above-describedobjects. FIG. 33G is a television receiver which can include a tuner, animage processing portion, and the like in addition to theabove-described objects. FIG. 33H illustrates a portable televisionreceiver which can include a charger 5017 which can transmit and receivesignals and the like in addition to the above objects. FIG. 34Aillustrates a display which can include a supporting board 5018 and thelike in addition to the above objects. FIG. 34B is a camera which caninclude an external connecting port 5019, a shutter button 5015, animage receiver portion 5016, and the like in addition to theabove-described objects. FIG. 34C is a computer which can include apointing device 5020, an external connecting port 5019, a reader/writer5021, and the like in addition to the above-described objects. FIG. 34Dillustrates a mobile phone which can include an antenna 5014, a tuner ofone-segment partial reception service for mobile phones and mobileterminals (“1seg”), and the like in addition to the above objects.

The electronic devices shown in FIGS. 33A to 33H and FIGS. 34A to 34Dcan have a variety of functions. For example, a function of displaying avariety of information (a still image, a moving image, a text image, andthe like) on a display portion, a touch panel function, a function ofdisplaying a calendar, date, time, and the like, a function ofcontrolling processing with a variety of software (programs), a wirelesscommunication function, a function of being connected to a variety ofcomputer networks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, and a function of reading program or data storedin a memory medium and displaying the program or data on a displayportion can be given. Further, the electronic device including aplurality of display portions can have a function of displaying imageinformation mainly on one display portion while displaying textinformation on another display portion, a function of displaying athree-dimensional image by displaying images where parallax isconsidered on a plurality of display portions, or the like. Furthermore,the electronic device including an image receiver portion can have afunction of shooting a still image, a function of shooting a movingimage, a function of automatically or manually correcting a shot image,a function of storing a shot image in a memory medium (an externalmemory medium or a memory medium incorporated in the camera), a functionof displaying a shot image on the display portion, or the like. Notethat functions which can be provided for the electronic devices shown inFIGS. 33A to 33H and FIGS. 34A to 34D are not limited thereto, and theelectronic devices can have a variety of functions.

Electronic devices described in this embodiment are characterized byhaving a display portion for displaying some sort of information. By acombination of the electronic device in this embodiment and thesemiconductor device, shift register, or display device in Embodiments 1to 9, improvement in reliability, improvement in yield, reduction incost, increase in the size of the display portion, increase in thedefinition of the display portion, or the like can be achieved.

Next, application examples of a semiconductor device are described.

FIG. 34E shows an example in which a semiconductor device is provided soas to be integrated with a building. In FIG. 34E, a housing 5022, adisplay portion 5023, a remote controller device 5024 which is anoperation portion, a speaker 5025, and the like are included. Thesemiconductor device is incorporated in the constructed object as awall-hanging type and can be provided without requiring a large space.

FIG. 34F illustrates an example where a semiconductor device isincorporated in a constructed object. The display panel 5026 isintegrated with a prefabricated bath 5027, so that a person who takes abath can watch the display panel 5026.

Note that although this embodiment gives the wall and the prefabricatedbath as examples of the building, this embodiment is not limited to themand the semiconductor device can be provided in a variety of buildings.

Next, examples where a semiconductor device is incorporated with amoving object are described.

FIG. 34G illustrates an example in which the semiconductor device isprovided in a vehicle. A display panel 5028 is provided in a body 5029of the vehicle and can display information input from the operation ofthe body or the outside of the body on demand. Note that a navigationfunction may be provided.

FIG. 34H shows an example in which the semiconductor device is providedso as to be integrated with a passenger airplane. FIG. 34H shows a usagepattern when a display panel 5031 is provided on a ceiling 5030 above aseat in the passenger airplane. The display panel 5031 is integratedwith the ceiling 5030 through a hinge portion 5032, and a passenger canwatch the display panel 5031 by extending and contracting the hingeportion 5032. The display panel 5031 has a function of displayinginformation when operated by the passenger.

Note that although this embodiment gives the body of the vehicle and thebody of the plane as examples of the moving body, this embodiment is notlimited thereto. The semiconductor device can be provided to a varietyof moving bodies such as a two-wheel motor vehicle, a four-wheel vehicle(including a car, bus, and the like), a train (including a monorail, arailway, and the like), and a ship.

This application is based on Japanese Patent Application serial No.2008-304124 filed with Japan Patent Office on Nov. 28, 2008, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a first transistor,a second transistor, and a third transistor each including a gate, afirst terminal, and a second terminal; a first conductive layerincluding the gate of the first transistor; a second conductive layerincluding the gate of the second transistor; a third conductive layerincluding the gate of the third transistor; a fourth conductive layerincluding the first terminal of the first transistor; a fifth conductivelayer including the first terminal of the second transistor, the fifthconductive layer including the first terminal of the third transistor;and a sixth conductive layer including the second terminal of the firsttransistor, the sixth conductive layer including the second terminal ofthe second transistor, and the sixth conductive layer including thesecond terminal of the third transistor, wherein the first conductivelayer and the sixth conductive layer overlap with each other in a firstregion, wherein the first conductive layer and the fourth conductivelayer overlap with each other in a second region, wherein the secondconductive layer and the sixth conductive layer overlap with each otherin a third region, wherein the second conductive layer and the fifthconductive layer overlap with each other in a fourth region, wherein thethird conductive layer and the sixth conductive layer overlap with eachother in a fifth region, wherein the third conductive layer and thefifth conductive layer overlap with each other in a sixth region,wherein area of the first region is larger than area of the secondregion, wherein area of the third region is smaller than area of thefourth region, and wherein area of the fifth region is smaller than areaof the sixth region.
 3. The semiconductor device according to claim 2,further comprising: a fourth transistor, a fifth transistor, a sixthtransistor, and seventh transistor each including a gate, a firstterminal, and a second terminal, wherein the gate of the fourthtransistor is electrically connected to the first terminal of the firsttransistor, the first terminal of the fourth transistor is electricallyconnected to the gate of the first transistor, and the second terminalof the fourth transistor is electrically connected to the secondterminal of the first transistor, wherein the first terminal of thefifth transistor is electrically connected to the gate of the firsttransistor, wherein the gate of the sixth transistor is electricallyconnected to the gate of the first transistor, the first terminal of thesixth transistor is electrically connected to the first terminal of thefifth transistor, and the second terminal of the sixth transistor iselectrically connected to the first terminal of the second transistor,and wherein the gate of the seventh transistor is electrically connectedto the gate of the third transistor, the first terminal of the seventhtransistor is electrically connected to the gate of the firsttransistor, and the second terminal of the seventh transistor iselectrically connected to the first terminal of the second transistor.4. The semiconductor device according to claim 2, further comprising: acapacitor including a first electrode and a second electrode, whereinthe first electrode is electrically connected to the gate of the firsttransistor, and wherein the second electrode is electrically connectedto the second terminal of the first transistor.
 5. The semiconductordevice according to claim 2, further comprising: a capacitor including afirst electrode and a second electrode, wherein the first electrode iselectrically connected to the first terminal of the first transistor. 6.The semiconductor device according to claim 2, wherein a channel regionof one of the first transistor, the second transistor and the thirdtransistor comprises an oxide semiconductor.
 7. The semiconductor deviceaccording to claim 2, wherein a channel region of one of the firsttransistor, the second transistor and the third transistor comprisesamorphous silicon.
 8. The semiconductor device according to claim 2,wherein a channel region of one of the first transistor, the secondtransistor and the third transistor comprises polycrystalline silicon.9. A display module comprising: the semiconductor device according toclaim 2; and an FPC.
 10. An electric apparatus comprising: thesemiconductor device according to claim 2; and at least one of anantenna, a microphone, a speaker, and a sensor.
 11. A shift registercomprising: a first transistor, a second transistor, and a thirdtransistor each including a gate, a first terminal, and a secondterminal; a first conductive layer including the gate of the firsttransistor; a second conductive layer including the gate of the secondtransistor; a third conductive layer including the gate of the thirdtransistor; a fourth conductive layer including the first terminal ofthe first transistor; a fifth conductive layer including the firstterminal of the second transistor, the fifth conductive layer includingthe first terminal of the third transistor; and a sixth conductive layerincluding the second terminal of the first transistor, the sixthconductive layer including the second terminal of the second transistor,and the sixth conductive layer including the second terminal of thethird transistor, wherein the first conductive layer and the sixthconductive layer overlap with each other in a first region, wherein thefirst conductive layer and the fourth conductive layer overlap with eachother in a second region, wherein the second conductive layer and thesixth conductive layer overlap with each other in a third region,wherein the second conductive layer and the fifth conductive layeroverlap with each other in a fourth region, wherein the third conductivelayer and the sixth conductive layer overlap with each other in a fifthregion, wherein the third conductive layer and the fifth conductivelayer overlap with each other in a sixth region, wherein area of thefirst region is larger than area of the second region, wherein area ofthe third region is smaller than area of the fourth region, and whereinarea of the fifth region is smaller than area of the sixth region. 12.The shift register according to claim 11, further comprising: a fourthtransistor, a fifth transistor, a sixth transistor, and seventhtransistor each including a gate, a first terminal, and a secondterminal, wherein the gate of the fourth transistor is electricallyconnected to the first terminal of the first transistor, the firstterminal of the fourth transistor is electrically connected to the gateof the first transistor, and the second terminal of the fourthtransistor is electrically connected to the second terminal of the firsttransistor, wherein the first terminal of the fifth transistor iselectrically connected to the gate of the first transistor, wherein thegate of the sixth transistor is electrically connected to the gate ofthe first transistor, the first terminal of the sixth transistor iselectrically connected to the first terminal of the fifth transistor,and the second terminal of the sixth transistor is electricallyconnected to the first terminal of the second transistor, and whereinthe gate of the seventh transistor is electrically connected to the gateof the third transistor, the first terminal of the seventh transistor iselectrically connected to the gate of the first transistor, and thesecond terminal of the seventh transistor is electrically connected tothe first terminal of the second transistor.
 13. The shift registeraccording to claim 11, further comprising: a capacitor including a firstelectrode and a second electrode, wherein the first electrode iselectrically connected to the gate of the first transistor, and whereinthe second electrode is electrically connected to the second terminal ofthe first transistor.
 14. The shift register according to claim 11,further comprising: a capacitor including a first electrode and a secondelectrode, wherein the first electrode is electrically connected to thefirst terminal of the first transistor.
 15. The shift register accordingto claim 11, wherein a channel region of one of the first transistor,the second transistor and the third transistor comprises an oxidesemiconductor.
 16. The shift register according to claim 11, wherein achannel region of one of the first transistor, the second transistor andthe third transistor comprises amorphous silicon.
 17. The shift registeraccording to claim 11, wherein a channel region of one of the firsttransistor, the second transistor and the third transistor comprisespolycrystalline silicon.
 18. A display module comprising: the shiftregister according to claim 11; and an FPC.
 19. An electric apparatuscomprising: the shift register according to claim 11; and at least oneof an antenna, a microphone, a speaker, and a sensor.